<s>
A	O
hard	B-Device
disk	I-Device
drive	I-Device
(	O
HDD	O
)	O
,	O
hard	B-Device
disk	I-Device
,	O
hard	B-Device
drive	I-Device
,	O
or	O
fixed	B-Device
disk	I-Device
,	O
is	O
an	O
electro-mechanical	O
data	B-General_Concept
storage	I-General_Concept
device	I-General_Concept
that	O
stores	O
and	O
retrieves	O
digital	B-General_Concept
data	I-General_Concept
using	O
magnetic	B-Architecture
storage	I-Architecture
with	O
one	O
or	O
more	O
rigid	O
rapidly	O
rotating	O
platters	B-Device
coated	O
with	O
magnetic	O
material	O
.	O
</s>
<s>
The	O
platters	B-Device
are	O
paired	O
with	O
magnetic	B-Device
heads	I-Device
,	O
usually	O
arranged	O
on	O
a	O
moving	O
actuator	B-Algorithm
arm	O
,	O
which	O
read	O
and	O
write	O
data	O
to	O
the	O
platter	O
surfaces	O
.	O
</s>
<s>
Data	O
is	O
accessed	O
in	O
a	O
random-access	B-General_Concept
manner	O
,	O
meaning	O
that	O
individual	O
blocks	B-General_Concept
of	O
data	O
can	O
be	O
stored	O
and	O
retrieved	O
in	O
any	O
order	O
.	O
</s>
<s>
HDDs	O
are	O
a	O
type	O
of	O
non-volatile	B-General_Concept
storage	I-General_Concept
,	O
retaining	O
stored	B-General_Concept
data	I-General_Concept
when	O
powered	O
off	O
.	O
</s>
<s>
Modern	O
HDDs	O
are	O
typically	O
in	O
the	O
form	O
of	O
a	O
small	O
rectangular	B-Device
box	I-Device
.	O
</s>
<s>
HDDs	O
maintained	O
this	O
position	O
into	O
the	O
modern	O
era	O
of	O
servers	B-Application
and	O
personal	B-Device
computers	I-Device
,	O
though	O
personal	B-Device
computing	I-Device
devices	O
produced	O
in	O
large	O
volume	O
,	O
like	O
cell	O
phones	O
and	O
tablets	O
,	O
rely	O
on	O
flash	B-Device
memory	I-Device
storage	O
devices	O
.	O
</s>
<s>
More	O
than	O
224	O
companies	O
have	O
produced	B-Device
HDDs	I-Device
historically	I-Device
,	O
though	O
after	O
extensive	O
industry	O
consolidation	O
most	O
units	O
are	O
manufactured	O
by	O
Seagate	O
,	O
Toshiba	O
,	O
and	O
Western	O
Digital	O
.	O
</s>
<s>
HDDs	O
dominate	O
the	O
volume	O
of	O
storage	O
produced	O
(	O
exabytes	O
per	O
year	O
)	O
for	O
servers	B-Application
.	O
</s>
<s>
Though	O
production	O
is	O
growing	O
slowly	O
(	O
by	O
exabytes	O
shipped	O
)	O
,	O
sales	O
revenues	O
and	O
unit	O
shipments	O
are	O
declining	O
because	O
solid-state	B-Device
drives	I-Device
(	O
SSDs	B-Device
)	O
have	O
higher	O
data-transfer	O
rates	O
,	O
higher	O
areal	B-Device
storage	I-Device
density	I-Device
,	O
somewhat	O
better	O
reliability	O
,	O
and	O
much	O
lower	O
latency	O
and	O
access	B-General_Concept
times	I-General_Concept
.	O
</s>
<s>
The	O
revenues	O
for	O
SSDs	B-Device
,	O
most	O
of	O
which	O
use	O
NAND	O
flash	B-Device
memory	I-Device
,	O
slightly	O
exceeded	O
those	O
for	O
HDDs	O
in	O
2018	O
.	O
</s>
<s>
Flash	B-Device
storage	I-Device
products	O
had	O
more	O
than	O
twice	O
the	O
revenue	O
of	O
hard	B-Device
disk	I-Device
drives	I-Device
.	O
</s>
<s>
Though	O
SSDs	B-Device
have	O
four	O
to	O
nine	O
times	O
higher	O
cost	O
per	O
bit	O
,	O
they	O
are	O
replacing	O
HDDs	O
in	O
applications	O
where	O
speed	O
,	O
power	O
consumption	O
,	O
small	O
size	O
,	O
high	O
capacity	O
and	O
durability	O
are	O
important	O
.	O
</s>
<s>
,	O
the	O
cost	O
per	O
bit	O
of	O
SSDs	B-Device
is	O
falling	O
,	O
and	O
the	O
price	O
premium	O
over	O
HDDs	O
has	O
narrowed	O
.	O
</s>
<s>
The	O
primary	O
characteristics	O
of	O
an	O
HDD	O
are	O
its	O
capacity	O
and	O
performance	B-Device
.	O
</s>
<s>
Capacity	O
is	O
specified	O
in	O
unit	O
prefixes	O
corresponding	O
to	O
powers	O
of	O
:	O
a	O
1-terabyte	O
(	O
TB	O
)	O
drive	O
has	O
a	O
capacity	O
of	O
gigabytes	O
(	O
GB	O
;	O
where	O
1	O
gigabyte	O
=	O
(	O
109	O
)	O
bytes	B-Application
)	O
.	O
</s>
<s>
Typically	O
,	O
some	O
of	O
an	O
HDD	O
's	O
capacity	O
is	O
unavailable	O
to	O
the	O
user	O
because	O
it	O
is	O
used	O
by	O
the	O
file	B-Application
system	I-Application
and	O
the	O
computer	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
,	O
and	O
possibly	O
inbuilt	O
redundancy	O
for	O
error	B-Error_Name
correction	I-Error_Name
and	O
recovery	O
.	O
</s>
<s>
There	O
can	O
be	O
confusion	O
regarding	O
storage	O
capacity	O
,	O
since	O
capacities	O
are	O
stated	O
in	O
decimal	O
gigabytes	O
(	O
powers	O
of	O
1000	O
)	O
by	O
HDD	O
manufacturers	O
,	O
whereas	O
the	O
most	O
commonly	O
used	O
operating	B-General_Concept
systems	I-General_Concept
report	O
capacities	O
in	O
powers	O
of	O
1024	O
,	O
which	O
results	O
in	O
a	O
smaller	O
number	O
than	O
advertised	O
.	O
</s>
<s>
Performance	B-Device
is	O
specified	O
as	O
the	O
time	O
required	O
to	O
move	O
the	O
heads	O
to	O
a	O
track	O
or	O
cylinder	O
(	O
average	O
access	B-General_Concept
time	I-General_Concept
)	O
,	O
the	O
time	O
it	O
takes	O
for	O
the	O
desired	O
sector	O
to	O
move	O
under	O
the	O
head	O
(	O
average	O
latency	O
,	O
which	O
is	O
a	O
function	O
of	O
the	O
physical	O
rotational	O
speed	O
in	O
revolutions	O
per	O
minute	O
)	O
,	O
and	O
finally	O
the	O
speed	O
at	O
which	O
the	O
data	O
is	O
transmitted	O
(	O
data	O
rate	O
)	O
.	O
</s>
<s>
The	O
two	O
most	O
common	O
form	O
factors	O
for	O
modern	O
HDDs	O
are	O
3.5-inch	O
,	O
for	O
desktop	B-Device
computers	I-Device
,	O
and	O
2.5-inch	O
,	O
primarily	O
for	O
laptops	O
.	O
</s>
<s>
HDDs	O
are	O
connected	O
to	O
systems	O
by	O
standard	O
interface	B-Application
cables	O
such	O
as	O
PATA	B-Protocol
(	O
Parallel	B-Protocol
ATA	I-Protocol
)	O
,	O
SATA	O
(	O
Serial	O
ATA	B-Protocol
)	O
,	O
USB	B-Protocol
or	O
SAS	O
(	O
Serial	O
Attached	O
SCSI	B-Architecture
)	O
cables	O
.	O
</s>
<s>
+	O
Improvement	O
of	O
HDD	O
characteristics	O
over	O
time	O
Parameter	O
Started	O
with	O
(	O
1957	O
)	O
Improved	O
to	O
Improvement	O
Capacity(formatted )	O
3.75	O
megabytes	O
22	O
terabytes	O
(	O
)	O
5.86-million-to-one	O
Physical	O
volume	O
56,000	O
-to-one	O
Weight	O
15,000	O
-to-one	O
Average	O
access	B-General_Concept
time	I-General_Concept
approx	O
.	O
</s>
<s>
The	O
first	O
production	O
IBM	O
hard	B-Device
disk	I-Device
drive	I-Device
,	O
the	O
350	B-Device
disk	I-Device
storage	I-Device
,	O
shipped	O
in	O
1957	O
as	O
a	O
component	O
of	O
the	O
IBM	O
305	O
RAMAC	O
system	O
.	O
</s>
<s>
It	O
was	O
approximately	O
the	O
size	O
of	O
two	O
medium-sized	O
refrigerators	O
and	O
stored	O
five	O
million	O
six-bit	O
characters	O
(	O
3.75	O
megabytes	O
)	O
on	O
a	O
stack	O
of	O
52	O
disks	B-Device
(	O
100	O
surfaces	O
used	O
)	O
.	O
</s>
<s>
The	O
350	O
had	O
a	O
single	O
arm	O
with	O
two	O
read/write	B-Device
heads	I-Device
,	O
one	O
facing	O
up	O
and	O
the	O
other	O
down	O
,	O
that	O
moved	O
both	O
horizontally	O
between	O
a	O
pair	O
of	O
adjacent	O
platters	B-Device
and	O
vertically	O
from	O
one	O
pair	O
of	O
platters	B-Device
to	O
a	O
second	O
set	O
.	O
</s>
<s>
Variants	O
of	O
the	O
IBM	B-Device
350	I-Device
were	O
the	O
IBM	O
355	O
,	O
IBM	O
7300	O
and	O
IBM	O
1405	O
.	O
</s>
<s>
The	O
1301	O
consisted	O
of	O
one	O
(	O
for	O
Model	O
1	O
)	O
or	O
two	O
(	O
for	O
model	O
2	O
)	O
modules	O
,	O
each	O
containing	O
25	O
platters	B-Device
,	O
each	O
platter	O
about	O
thick	O
and	O
in	O
diameter	O
.	O
</s>
<s>
While	O
the	O
earlier	O
IBM	O
disk	B-General_Concept
drives	I-General_Concept
used	O
only	O
two	O
read/write	B-Device
heads	I-Device
per	O
arm	O
,	O
the	O
1301	O
used	O
an	O
array	O
of	O
48	O
heads	O
(	O
comb	O
)	O
,	O
each	O
array	O
moving	O
horizontally	O
as	O
a	O
single	O
unit	O
,	O
one	O
head	O
per	O
surface	O
used	O
.	O
</s>
<s>
Cylinder-mode	B-Device
read/write	O
operations	O
were	O
supported	O
,	O
and	O
the	O
heads	O
flew	O
about	O
250	O
micro-inches	O
(	O
about	O
6µm	O
)	O
above	O
the	O
platter	O
surface	O
.	O
</s>
<s>
Motion	O
of	O
the	O
head	O
array	O
depended	O
upon	O
a	O
binary	O
adder	O
system	O
of	O
hydraulic	O
actuators	B-Algorithm
which	O
assured	O
repeatable	O
positioning	O
.	O
</s>
<s>
The	O
1301	O
cabinet	O
was	O
about	O
the	O
size	O
of	O
three	O
home	O
refrigerators	O
placed	O
side	O
by	O
side	O
,	O
storing	O
the	O
equivalent	O
of	O
about	O
21	O
million	O
eight-bit	O
bytes	B-Application
per	O
module	O
.	O
</s>
<s>
Access	B-General_Concept
time	I-General_Concept
was	O
about	O
a	O
quarter	O
of	O
a	O
second	O
.	O
</s>
<s>
Also	O
in	O
1962	O
,	O
IBM	O
introduced	O
the	O
model	B-Device
1311	I-Device
disk	B-Device
drive	I-Device
,	O
which	O
was	O
about	O
the	O
size	O
of	O
a	O
washing	O
machine	O
and	O
stored	O
two	O
million	O
characters	O
on	O
a	O
removable	O
disk	B-Operating_System
pack	I-Operating_System
.	O
</s>
<s>
Non-removable	O
HDDs	O
were	O
called	O
"	O
fixed	B-Device
disk	I-Device
"	O
drives	O
.	O
</s>
<s>
Some	O
high-performance	O
HDDs	O
were	O
manufactured	O
with	O
one	O
head	O
per	O
track	O
,	O
e.g.	O
,	O
Burroughs	O
B-475	O
in	O
1964	O
,	O
IBM	O
2305	O
in	O
1970	O
,	O
so	O
that	O
no	O
time	O
was	O
lost	O
physically	O
moving	O
the	O
heads	O
to	O
a	O
track	O
and	O
the	O
only	O
latency	O
was	O
the	O
time	O
for	O
the	O
desired	O
block	O
of	O
data	O
to	O
rotate	O
into	O
position	O
under	O
the	O
head	O
.	O
</s>
<s>
Known	O
as	O
fixed-head	O
or	O
head-per-track	O
disk	B-General_Concept
drives	I-General_Concept
,	O
they	O
were	O
very	O
expensive	O
and	O
are	O
no	O
longer	O
in	O
production	O
.	O
</s>
<s>
In	O
1973	O
,	O
IBM	O
introduced	O
a	O
new	O
type	O
of	O
HDD	O
code-named	O
"	O
Winchester	B-Device
"	O
.	O
</s>
<s>
Its	O
primary	O
distinguishing	O
feature	O
was	O
that	O
the	O
disk	B-Device
heads	I-Device
were	O
not	O
withdrawn	O
completely	O
from	O
the	O
stack	O
of	O
disk	B-Device
platters	I-Device
when	O
the	O
drive	O
was	O
powered	O
down	O
.	O
</s>
<s>
Instead	O
,	O
the	O
heads	O
were	O
allowed	O
to	O
"	O
land	O
"	O
on	O
a	O
special	O
area	O
of	O
the	O
disk	B-Device
surface	O
upon	O
spin-down	O
,	O
"	O
taking	O
off	O
"	O
again	O
when	O
the	O
disk	B-Device
was	O
later	O
powered	O
on	O
.	O
</s>
<s>
This	O
greatly	O
reduced	O
the	O
cost	O
of	O
the	O
head	O
actuator	B-Algorithm
mechanism	O
,	O
but	O
precluded	O
removing	O
just	O
the	O
disks	B-Device
from	O
the	O
drive	O
as	O
was	O
done	O
with	O
the	O
disk	B-Operating_System
packs	I-Operating_System
of	O
the	O
day	O
.	O
</s>
<s>
Instead	O
,	O
the	O
first	O
models	O
of	O
"	O
Winchester	B-Device
technology	O
"	O
drives	O
featured	O
a	O
removable	O
disk	B-Device
module	O
,	O
which	O
included	O
both	O
the	O
disk	B-Operating_System
pack	I-Operating_System
and	O
the	O
head	O
assembly	O
,	O
leaving	O
the	O
actuator	B-Algorithm
motor	O
in	O
the	O
drive	O
upon	O
removal	O
.	O
</s>
<s>
Later	O
"	O
Winchester	B-Device
"	O
drives	O
abandoned	O
the	O
removable	O
media	O
concept	O
and	O
returned	O
to	O
non-removable	O
platters	B-Device
.	O
</s>
<s>
In	O
1974	O
IBM	O
introduced	O
the	O
swinging	O
arm	O
actuator	B-Algorithm
,	O
made	O
feasible	O
because	O
the	O
Winchester	B-Device
recording	O
heads	O
function	O
well	O
when	O
skewed	O
to	O
the	O
recorded	O
tracks	O
.	O
</s>
<s>
The	O
simple	O
design	O
of	O
the	O
IBM	O
GV	O
(	O
Gulliver	O
)	O
drive	O
,	O
invented	O
at	O
IBM	O
's	O
UK	O
Hursley	O
Labs	O
,	O
became	O
IBM	O
's	O
most	O
licensed	O
electro-mechanical	O
invention	O
of	O
all	O
time	O
,	O
the	O
actuator	B-Algorithm
and	O
filtration	O
system	O
being	O
adopted	O
in	O
the	O
1980s	O
eventually	O
for	O
all	O
HDDs	O
,	O
and	O
still	O
universal	O
nearly	O
40	O
years	O
and	O
10	O
Billion	O
arms	O
later	O
.	O
</s>
<s>
Like	O
the	O
first	O
removable	O
pack	O
drive	O
,	O
the	O
first	O
"	O
Winchester	B-Device
"	O
drives	O
used	O
platters	B-Device
in	O
diameter	O
.	O
</s>
<s>
In	O
1978	O
IBM	O
introduced	O
a	O
swing	O
arm	O
drive	O
,	O
the	O
IBM	O
0680	O
(	O
Piccolo	O
)	O
,	O
with	O
eight	O
inch	O
platters	B-Device
,	O
exploring	O
the	O
possibility	O
that	O
smaller	O
platters	B-Device
might	O
offer	O
advantages	O
.	O
</s>
<s>
Other	O
eight	O
inch	O
drives	O
followed	O
,	O
then	O
drives	O
,	O
sized	O
to	O
replace	O
the	O
contemporary	O
floppy	B-Device
disk	I-Device
drives	I-Device
.	O
</s>
<s>
The	O
latter	O
were	O
primarily	O
intended	O
for	O
the	O
then	O
fledgling	O
personal	B-Device
computer	I-Device
(	O
PC	O
)	O
market	O
.	O
</s>
<s>
Over	O
time	O
,	O
as	O
recording	O
densities	O
were	O
greatly	O
increased	O
,	O
further	O
reductions	O
in	O
disk	B-Device
diameter	O
to	O
3.5	B-Device
"	I-Device
and	O
2.5	O
"	O
were	O
found	O
to	O
be	O
optimum	O
.	O
</s>
<s>
Powerful	O
rare	O
earth	O
magnet	O
materials	O
became	O
affordable	O
during	O
this	O
period	O
,	O
and	O
were	O
complementary	O
to	O
the	O
swing	O
arm	O
actuator	B-Algorithm
design	O
to	O
make	O
possible	O
the	O
compact	O
form	O
factors	O
of	O
modern	O
HDDs	O
.	O
</s>
<s>
As	O
the	O
1980s	O
began	O
,	O
HDDs	O
were	O
a	O
rare	O
and	O
very	O
expensive	O
additional	O
feature	O
in	O
PCs	B-Device
,	O
but	O
by	O
the	O
late	O
1980s	O
their	O
cost	O
had	O
been	O
reduced	O
to	O
the	O
point	O
where	O
they	O
were	O
standard	O
on	O
all	O
but	O
the	O
cheapest	O
computers	O
.	O
</s>
<s>
The	O
subsystem	O
was	O
not	O
sold	O
under	O
the	O
drive	O
manufacturer	O
's	O
name	O
but	O
under	O
the	O
subsystem	O
manufacturer	O
's	O
name	O
such	O
as	O
Corvus	O
Systems	O
and	O
Tallgrass	O
Technologies	O
,	O
or	O
under	O
the	O
PC	O
system	O
manufacturer	O
's	O
name	O
such	O
as	O
the	O
Apple	B-Device
ProFile	I-Device
.	O
</s>
<s>
The	O
IBM	B-Device
PC/XT	I-Device
in	O
1983	O
included	O
an	O
internal	O
10MB	O
HDD	O
,	O
and	O
soon	O
thereafter	O
internal	O
HDDs	O
proliferated	O
on	O
personal	B-Device
computers	I-Device
.	O
</s>
<s>
External	O
HDDs	O
remained	O
popular	O
for	O
much	O
longer	O
on	O
the	O
Apple	B-Device
Macintosh	I-Device
.	O
</s>
<s>
Many	O
Macintosh	B-Device
computers	I-Device
made	O
between	O
1986	O
and	O
1998	O
featured	O
a	O
SCSI	B-Architecture
port	O
on	O
the	O
back	O
,	O
making	O
external	O
expansion	O
simple	O
.	O
</s>
<s>
Older	O
compact	O
Macintosh	B-Device
computers	I-Device
did	O
not	O
have	O
user-accessible	O
hard	B-Device
drive	I-Device
bays	O
(	O
indeed	O
,	O
the	O
Macintosh	B-Device
128K	I-Device
,	O
Macintosh	B-Device
512K	I-Device
,	O
and	O
Macintosh	B-Device
Plus	I-Device
did	O
not	O
feature	O
a	O
hard	B-Device
drive	I-Device
bay	O
at	O
all	O
)	O
,	O
so	O
on	O
those	O
models	O
external	O
SCSI	B-Architecture
disks	B-Device
were	O
the	O
only	O
reasonable	O
option	O
for	O
expanding	O
upon	O
any	O
internal	O
storage	O
.	O
</s>
<s>
HDD	O
improvements	O
have	O
been	O
driven	O
by	O
increasing	O
areal	B-Device
density	I-Device
,	O
listed	O
in	O
the	O
table	O
above	O
.	O
</s>
<s>
Applications	O
expanded	O
through	O
the	O
2000s	O
,	O
from	O
the	O
mainframe	B-Architecture
computers	I-Architecture
of	O
the	O
late	O
1950s	O
to	O
most	O
mass	B-Device
storage	I-Device
applications	O
including	O
computers	O
and	O
consumer	O
applications	O
such	O
as	O
storage	O
of	O
entertainment	O
content	O
.	O
</s>
<s>
In	O
the	O
2000s	O
and	O
2010s	O
,	O
NAND	O
began	O
supplanting	O
HDDs	O
in	O
applications	O
requiring	O
portability	O
or	O
high	O
performance	B-Device
.	O
</s>
<s>
NAND	O
performance	B-Device
is	O
improving	O
faster	O
than	O
HDDs	O
,	O
and	O
applications	O
for	O
HDDs	O
are	O
eroding	O
.	O
</s>
<s>
In	O
2018	O
,	O
the	O
largest	O
hard	B-Device
drive	I-Device
had	O
a	O
capacity	O
of	O
15TB	O
,	O
while	O
the	O
largest	O
capacity	O
SSD	B-Device
had	O
a	O
capacity	O
of	O
100TB	O
.	O
</s>
<s>
The	O
2011	O
Thailand	O
floods	O
damaged	O
the	O
manufacturing	O
plants	O
and	O
impacted	O
hard	B-Device
disk	I-Device
drive	I-Device
cost	O
adversely	O
between	O
2011	O
and	O
2013	O
.	O
</s>
<s>
In	O
2019	O
,	O
Western	O
Digital	O
closed	O
its	O
last	O
Malaysian	O
HDD	O
factory	O
due	O
to	O
decreasing	O
demand	O
,	O
to	O
focus	O
on	O
SSD	B-Device
production	O
.	O
</s>
<s>
A	O
modern	O
HDD	O
records	O
data	O
by	O
magnetizing	O
a	O
thin	O
film	O
of	O
ferromagnetic	O
material	O
on	O
both	O
sides	O
of	O
a	O
disk	B-Device
.	O
</s>
<s>
The	O
data	O
is	O
read	O
from	O
the	O
disk	B-Device
by	O
detecting	O
the	O
transitions	O
in	O
magnetization	O
.	O
</s>
<s>
User	O
data	O
is	O
encoded	O
using	O
an	O
encoding	O
scheme	O
,	O
such	O
as	O
run-length	B-Protocol
limited	I-Protocol
encoding	I-Protocol
,	O
which	O
determines	O
how	O
the	O
data	O
is	O
represented	O
by	O
the	O
magnetic	O
transitions	O
.	O
</s>
<s>
A	O
typical	O
HDD	O
design	O
consists	O
of	O
a	O
that	O
holds	O
flat	O
circular	O
disks	B-Device
,	O
called	O
platters	B-Device
,	O
which	O
hold	O
the	O
recorded	O
data	O
.	O
</s>
<s>
The	O
platters	B-Device
are	O
made	O
from	O
a	O
non-magnetic	O
material	O
,	O
usually	O
aluminum	O
alloy	O
,	O
glass	O
,	O
or	O
ceramic	O
.	O
</s>
<s>
The	O
platters	B-Device
in	O
contemporary	O
HDDs	O
are	O
spun	O
at	O
speeds	O
varying	O
from	O
4,200	O
RPM	O
in	O
energy-efficient	O
portable	O
devices	O
,	O
to	O
15,000	O
rpm	O
for	O
high-performance	O
servers	B-Application
.	O
</s>
<s>
,	O
the	O
platters	B-Device
in	O
most	O
consumer-grade	O
HDDs	O
spin	O
at	O
5,400	O
or	O
7,200	O
RPM	O
.	O
</s>
<s>
Information	O
is	O
written	O
to	O
and	O
read	O
from	O
a	O
platter	O
as	O
it	O
rotates	O
past	O
devices	O
called	O
read-and-write	B-Device
heads	I-Device
that	O
are	O
positioned	O
to	O
operate	O
very	O
close	O
to	O
the	O
magnetic	O
surface	O
,	O
with	O
their	O
flying	B-Device
height	I-Device
often	O
in	O
the	O
range	O
of	O
tens	O
of	O
nanometers	O
.	O
</s>
<s>
An	O
actuator	B-Algorithm
arm	O
(	O
or	O
access	B-Device
arm	I-Device
)	O
moves	O
the	O
heads	O
on	O
an	O
arc	O
(	O
roughly	O
radially	O
)	O
across	O
the	O
platters	B-Device
as	O
they	O
spin	O
,	O
allowing	O
each	O
head	O
to	O
access	O
almost	O
the	O
entire	O
surface	O
of	O
the	O
platter	O
as	O
it	O
spins	O
.	O
</s>
<s>
The	O
arm	O
is	O
moved	O
using	O
a	O
voice	B-Device
coil	I-Device
actuator	B-Algorithm
or	O
in	O
some	O
older	O
designs	O
a	O
stepper	B-Algorithm
motor	I-Algorithm
.	O
</s>
<s>
Early	O
hard	B-Device
disk	I-Device
drives	I-Device
wrote	O
data	O
at	O
some	O
constant	O
bits	O
per	O
second	O
,	O
resulting	O
in	O
all	O
tracks	O
having	O
the	O
same	O
amount	O
of	O
data	O
per	O
track	O
but	O
modern	O
drives	O
(	O
since	O
the	O
1990s	O
)	O
use	O
zone	B-Architecture
bit	I-Architecture
recording	I-Architecture
–	O
increasing	O
the	O
write	O
speed	O
from	O
inner	O
to	O
outer	O
zone	O
and	O
thereby	O
storing	O
more	O
data	O
per	O
track	O
in	O
the	O
outer	O
zones	B-Architecture
.	O
</s>
<s>
To	O
counter	O
this	O
,	O
the	O
platters	B-Device
are	O
coated	O
with	O
two	O
parallel	O
magnetic	O
layers	O
,	O
separated	O
by	O
a	O
three-atom	O
layer	O
of	O
the	O
non-magnetic	O
element	O
ruthenium	O
,	O
and	O
the	O
two	O
layers	O
are	O
magnetized	O
in	O
opposite	O
orientation	O
,	O
thus	O
reinforcing	O
each	O
other	O
.	O
</s>
<s>
Another	O
technology	O
used	O
to	O
overcome	O
thermal	O
effects	O
to	O
allow	O
greater	O
recording	O
densities	O
is	O
perpendicular	B-Device
recording	I-Device
,	O
first	O
shipped	O
in	O
2005	O
,	O
and	O
used	O
in	O
certain	O
HDDs	O
.	O
</s>
<s>
In	O
2004	O
,	O
a	O
higher-density	O
recording	B-General_Concept
media	I-General_Concept
was	O
introduced	O
,	O
consisting	O
of	O
coupled	O
soft	O
and	O
hard	O
magnetic	O
layers	O
.	O
</s>
<s>
So-called	O
exchange	B-General_Concept
spring	I-General_Concept
media	I-General_Concept
magnetic	B-Architecture
storage	I-Architecture
technology	O
,	O
also	O
known	O
as	O
exchange	B-General_Concept
coupled	I-General_Concept
composite	I-General_Concept
media	I-General_Concept
,	O
allows	O
good	O
writability	O
due	O
to	O
the	O
write-assist	O
nature	O
of	O
the	O
soft	O
layer	O
.	O
</s>
<s>
A	O
typical	O
HDD	O
has	O
two	O
electric	O
motors	O
:	O
a	O
spindle	O
motor	O
that	O
spins	O
the	O
disks	B-Device
and	O
an	O
actuator	B-Algorithm
(	O
motor	O
)	O
that	O
positions	O
the	O
read/write	B-Device
head	I-Device
assembly	O
across	O
the	O
spinning	O
disks	B-Device
.	O
</s>
<s>
The	O
disk	B-Device
motor	O
has	O
an	O
external	O
rotor	O
attached	O
to	O
the	O
disks	B-Device
;	O
the	O
stator	O
windings	O
are	O
fixed	O
in	O
place	O
.	O
</s>
<s>
Opposite	O
the	O
actuator	B-Algorithm
at	O
the	O
end	O
of	O
the	O
head	O
support	O
arm	O
is	O
the	O
read-write	B-Device
head	I-Device
;	O
thin	O
printed-circuit	O
cables	O
connect	O
the	O
read-write	B-Device
heads	I-Device
to	O
amplifier	O
electronics	O
mounted	O
at	O
the	O
pivot	O
of	O
the	O
actuator	B-Algorithm
.	O
</s>
<s>
Beneath	O
this	O
plate	O
is	O
the	O
moving	O
coil	O
,	O
often	O
referred	O
to	O
as	O
the	O
voice	B-Device
coil	I-Device
by	O
analogy	O
to	O
the	O
coil	O
in	O
loudspeakers	O
,	O
which	O
is	O
attached	O
to	O
the	O
actuator	B-Algorithm
hub	O
,	O
and	O
beneath	O
that	O
is	O
a	O
second	O
NIB	O
magnet	O
,	O
mounted	O
on	O
the	O
bottom	O
plate	O
of	O
the	O
motor	O
(	O
some	O
drives	O
have	O
only	O
one	O
magnet	O
)	O
.	O
</s>
<s>
The	O
voice	B-Device
coil	I-Device
itself	O
is	O
shaped	O
rather	O
like	O
an	O
arrowhead	O
and	O
is	O
made	O
of	O
doubly	O
coated	O
copper	O
magnet	O
wire	O
.	O
</s>
<s>
The	O
portions	O
of	O
the	O
coil	O
along	O
the	O
two	O
sides	O
of	O
the	O
arrowhead	O
(	O
which	O
point	O
to	O
the	O
center	O
of	O
the	O
actuator	B-Algorithm
bearing	O
)	O
then	O
interact	O
with	O
the	O
magnetic	O
field	O
of	O
the	O
fixed	O
magnet	O
.	O
</s>
<s>
The	O
HDD	O
's	O
electronics	O
control	O
the	O
movement	O
of	O
the	O
actuator	B-Algorithm
and	O
the	O
rotation	O
of	O
the	O
disk	B-Device
and	O
perform	O
reads	O
and	O
writes	O
on	O
demand	O
from	O
the	O
disk	B-General_Concept
controller	I-General_Concept
.	O
</s>
<s>
Feedback	O
of	O
the	O
drive	O
electronics	O
is	O
accomplished	O
by	O
means	O
of	O
special	O
segments	O
of	O
the	O
disk	B-Device
dedicated	O
to	O
servo	B-Algorithm
feedback	O
.	O
</s>
<s>
These	O
are	O
either	O
complete	O
concentric	O
circles	O
(	O
in	O
the	O
case	O
of	O
dedicated	O
servo	B-Algorithm
technology	O
)	O
or	O
segments	O
interspersed	O
with	O
real	O
data	O
(	O
in	O
the	O
case	O
of	O
embedded	O
servo	B-Algorithm
,	O
otherwise	O
known	O
as	O
sector	O
servo	B-Algorithm
technology	O
)	O
.	O
</s>
<s>
The	O
servo	B-Algorithm
feedback	O
optimizes	O
the	O
signal-to-noise	O
ratio	O
of	O
the	O
GMR	B-General_Concept
sensors	O
by	O
adjusting	O
the	O
voice	B-Device
coil	I-Device
motor	O
to	O
rotate	O
the	O
arm	O
.	O
</s>
<s>
A	O
more	O
modern	O
servo	B-Algorithm
system	O
also	O
employs	O
milli	O
and/or	O
micro	O
actuators	B-Algorithm
to	O
more	O
accurately	O
position	O
the	O
read/write	B-Device
heads	I-Device
.	O
</s>
<s>
The	O
spinning	O
of	O
the	O
disks	B-Device
uses	O
fluid-bearing	O
spindle	O
motors	O
.	O
</s>
<s>
Modern	O
disk	B-Device
firmware	O
is	O
capable	O
of	O
scheduling	O
reads	O
and	O
writes	O
efficiently	O
on	O
the	O
platter	O
surfaces	O
and	O
remapping	O
sectors	O
of	O
the	O
media	O
that	O
have	O
failed	O
.	O
</s>
<s>
Modern	O
drives	O
make	O
extensive	O
use	O
of	O
error	B-Error_Name
correction	I-Error_Name
codes	I-Error_Name
(	O
ECCs	O
)	O
,	O
particularly	O
Reed	B-Error_Name
–	I-Error_Name
Solomon	I-Error_Name
error	I-Error_Name
correction	I-Error_Name
.	O
</s>
<s>
For	O
example	O
,	O
a	O
typical	O
1TB	O
hard	B-Device
disk	I-Device
with	O
512-byte	O
sectors	O
provides	O
additional	O
capacity	O
of	O
about	O
93GB	O
for	O
the	O
ECC	B-Error_Name
data	O
.	O
</s>
<s>
In	O
the	O
newest	O
drives	O
,	O
,	O
low-density	B-Error_Name
parity-check	I-Error_Name
codes	I-Error_Name
(	O
LDPC	B-Error_Name
)	O
were	O
supplanting	O
Reed	B-Error_Name
–	I-Error_Name
Solomon	I-Error_Name
;	O
LDPC	B-Error_Name
codes	I-Error_Name
enable	O
performance	B-Device
close	O
to	O
the	O
Shannon	O
Limit	O
and	O
thus	O
provide	O
the	O
highest	O
storage	B-Device
density	I-Device
available	O
.	O
</s>
<s>
Typical	O
hard	B-Device
disk	I-Device
drives	I-Device
attempt	O
to	O
"	O
remap	O
"	O
the	O
data	O
in	O
a	O
physical	O
sector	O
that	O
is	O
failing	O
to	O
a	O
spare	O
physical	O
sector	O
provided	O
by	O
the	O
drive	O
's	O
"	O
spare	O
sector	O
pool	O
"	O
(	O
also	O
called	O
"	O
reserve	O
pool	O
"	O
)	O
,	O
while	O
relying	O
on	O
the	O
ECC	B-Error_Name
to	O
recover	O
stored	B-General_Concept
data	I-General_Concept
while	O
the	O
number	O
of	O
errors	O
in	O
a	O
bad	B-Device
sector	I-Device
is	O
still	O
low	O
enough	O
.	O
</s>
<s>
The	O
S.M.A.R.T	O
(	O
Self-Monitoring	O
,	O
Analysis	O
and	O
Reporting	O
Technology	O
)	O
feature	O
counts	O
the	O
total	O
number	O
of	O
errors	O
in	O
the	O
entire	O
HDD	O
fixed	O
by	O
ECC	B-Error_Name
(	O
although	O
not	O
on	O
all	O
hard	B-Device
drives	I-Device
as	O
the	O
related	O
S.M.A.R.T	O
attributes	O
"	O
Hardware	O
ECC	B-Error_Name
Recovered	O
"	O
and	O
"	O
Soft	O
ECC	B-Error_Name
Correction	O
"	O
are	O
not	O
consistently	O
supported	O
)	O
,	O
and	O
the	O
total	O
number	O
of	O
performed	O
sector	O
remappings	O
,	O
as	O
the	O
occurrence	O
of	O
many	O
such	O
errors	O
may	O
predict	O
an	O
HDD	B-Device
failure	I-Device
.	O
</s>
<s>
2013	O
specifications	O
for	O
enterprise	O
SAS	O
disk	B-General_Concept
drives	I-General_Concept
state	O
the	O
error	O
rate	O
to	O
be	O
one	O
uncorrected	O
bit	O
read	O
error	O
in	O
every	O
1016	O
bits	O
read	O
,	O
</s>
<s>
2018	O
specifications	O
for	O
consumer	O
SATA	O
hard	B-Device
drives	I-Device
state	O
the	O
error	O
rate	O
to	O
be	O
one	O
uncorrected	O
bit	O
read	O
error	O
in	O
every	O
1014	O
bits	O
.	O
</s>
<s>
The	O
worst	O
type	O
of	O
errors	O
are	O
silent	O
data	O
corruptions	O
which	O
are	O
errors	O
undetected	O
by	O
the	O
disk	B-Device
firmware	O
or	O
the	O
host	O
operating	B-General_Concept
system	I-General_Concept
;	O
some	O
of	O
these	O
errors	O
may	O
be	O
caused	O
by	O
hard	B-Device
disk	I-Device
drive	I-Device
malfunctions	O
while	O
others	O
originate	O
elsewhere	O
in	O
the	O
connection	O
between	O
the	O
drive	O
and	O
the	O
host	O
.	O
</s>
<s>
The	O
rate	O
of	O
areal	B-Device
density	I-Device
advancement	O
was	O
similar	O
to	O
Moore	O
's	O
law	O
(	O
doubling	O
every	O
two	O
years	O
)	O
through	O
2010	O
:	O
60%	O
per	O
year	O
during	O
1988	O
–	O
1996	O
,	O
100%	O
during	O
1996	O
–	O
2003	O
and	O
30%	O
during	O
2003	O
–	O
2010	O
.	O
</s>
<s>
Price	O
improvement	O
decelerated	O
to	O
−12%	O
per	O
year	O
during	O
2010	O
–	O
2017	O
,	O
as	O
the	O
growth	O
of	O
areal	B-Device
density	I-Device
slowed	O
.	O
</s>
<s>
The	O
rate	O
of	O
advancement	O
for	O
areal	B-Device
density	I-Device
slowed	O
to	O
10%	O
per	O
year	O
during	O
2010	O
–	O
2016	O
,	O
and	O
there	O
was	O
difficulty	O
in	O
migrating	O
from	O
perpendicular	B-Device
recording	I-Device
to	O
newer	O
technologies	O
.	O
</s>
<s>
As	O
bit	O
cell	O
size	O
decreases	O
,	O
more	O
data	O
can	O
be	O
put	O
onto	O
a	O
single	O
drive	B-Device
platter	I-Device
.	O
</s>
<s>
In	O
2013	O
,	O
a	O
production	O
desktop	B-Device
3TB	O
HDD	O
(	O
with	O
four	O
platters	B-Device
)	O
would	O
have	O
had	O
an	O
areal	B-Device
density	I-Device
of	O
about	O
500Gbit/in2	O
which	O
would	O
have	O
amounted	O
to	O
a	O
bit	O
cell	O
comprising	O
about	O
18	O
magnetic	O
grains	O
(	O
11	O
by	O
1.6	O
grains	O
)	O
.	O
</s>
<s>
Since	O
the	O
mid-2000s	O
areal	B-Device
density	I-Device
progress	O
has	O
been	O
challenged	O
by	O
a	O
superparamagnetic	O
trilemma	O
involving	O
grain	O
size	O
,	O
grain	O
magnetic	O
strength	O
and	O
ability	O
of	O
the	O
head	O
to	O
write	O
.	O
</s>
<s>
Magnetic	B-Architecture
storage	I-Architecture
technologies	O
are	O
being	O
developed	O
to	O
address	O
this	O
trilemma	O
,	O
and	O
compete	O
with	O
flash	B-Device
memory	I-Device
–	O
based	O
solid-state	B-Device
drives	I-Device
(	O
SSDs	B-Device
)	O
.	O
</s>
<s>
In	O
2013	O
,	O
Seagate	O
introduced	O
shingled	B-Device
magnetic	I-Device
recording	I-Device
(	O
SMR	O
)	O
,	O
intended	O
as	O
something	O
of	O
a	O
"	O
stopgap	O
"	O
technology	O
between	O
PMR	O
and	O
Seagate	O
's	O
intended	O
successor	O
heat-assisted	O
magnetic	B-Architecture
recording	I-Architecture
(	O
HAMR	O
)	O
,	O
SMR	O
utilises	O
overlapping	O
tracks	O
for	O
increased	O
data	B-Device
density	I-Device
,	O
at	O
the	O
cost	O
of	O
design	O
complexity	O
and	O
lower	O
data	O
access	O
speeds	O
(	O
particularly	O
write	O
speeds	O
and	O
random	B-General_Concept
access	I-General_Concept
4k	O
speeds	O
)	O
.	O
</s>
<s>
Since	O
turbulence	O
and	O
friction	O
are	O
reduced	O
,	O
higher	O
areal	B-Device
densities	I-Device
can	O
be	O
achieved	O
due	O
to	O
using	O
a	O
smaller	O
track	O
width	O
,	O
and	O
the	O
energy	O
dissipated	O
due	O
to	O
friction	O
is	O
lower	O
as	O
well	O
,	O
resulting	O
in	O
a	O
lower	O
power	O
draw	O
.	O
</s>
<s>
Furthermore	O
,	O
more	O
platters	B-Device
can	O
be	O
fit	O
into	O
the	O
same	O
enclosure	B-Device
space	O
,	O
although	O
helium	O
gas	O
is	O
notoriously	O
difficult	O
to	O
prevent	O
escaping	O
.	O
</s>
<s>
Other	O
recording	O
technologies	O
are	O
either	O
under	O
research	O
or	O
have	O
been	O
commercially	O
implemented	O
to	O
increase	O
areal	B-Device
density	I-Device
,	O
including	O
Seagate	O
's	O
heat-assisted	O
magnetic	B-Architecture
recording	I-Architecture
(	O
HAMR	O
)	O
.	O
</s>
<s>
HAMR	O
requires	O
a	O
different	O
architecture	O
with	O
redesigned	O
media	O
and	O
read/write	B-Device
heads	I-Device
,	O
new	O
lasers	O
,	O
and	O
new	O
near-field	O
optical	O
transducers	O
.	O
</s>
<s>
Some	O
drives	O
have	O
adopted	O
dual	O
independent	O
actuator	B-Algorithm
arms	O
to	O
increase	O
read/write	O
speeds	O
and	O
compete	O
with	O
SSDs	B-Device
.	O
</s>
<s>
Western	O
Digital	O
's	O
microwave-assisted	O
magnetic	B-Architecture
recording	I-Architecture
(	O
MAMR	O
)	O
,	O
also	O
referred	O
to	O
as	O
energy-assisted	O
magnetic	B-Architecture
recording	I-Architecture
(	O
EAMR	O
)	O
,	O
was	O
sampled	O
in	O
2020	O
,	O
with	O
the	O
first	O
EAMR	O
drive	O
,	O
the	O
Ultrastar	O
HC550	O
,	O
shipping	O
in	O
late	O
2020	O
.	O
</s>
<s>
Two-dimensional	B-Architecture
magnetic	I-Architecture
recording	I-Architecture
(	O
TDMR	B-Architecture
)	O
and	O
"	O
current	O
perpendicular	O
to	O
plane	O
"	O
giant	B-General_Concept
magnetoresistance	I-General_Concept
(	O
CPP/GMR	O
)	O
heads	O
have	O
appeared	O
in	O
research	O
papers	O
.	O
</s>
<s>
A	O
3D-actuated	O
vacuum	O
drive	O
(	O
3DHD	O
)	O
concept	O
has	O
been	O
proposed	O
.	O
</s>
<s>
Depending	O
upon	O
assumptions	O
on	O
feasibility	O
and	O
timing	O
of	O
these	O
technologies	O
,	O
Seagate	O
forecasts	O
that	O
areal	B-Device
density	I-Device
will	O
grow	O
20%	O
per	O
year	O
during	O
2020	O
–	O
2034	O
.	O
</s>
<s>
The	O
capacity	O
of	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
,	O
as	O
reported	O
by	O
an	O
operating	B-General_Concept
system	I-General_Concept
to	O
the	O
end	O
user	O
,	O
is	O
smaller	O
than	O
the	O
amount	O
stated	O
by	O
the	O
manufacturer	O
for	O
several	O
reasons	O
,	O
e.g.	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
using	O
some	O
space	O
,	O
use	O
of	O
some	O
space	O
for	O
data	O
redundancy	O
,	O
space	O
use	O
for	O
file	B-Application
system	I-Application
structures	O
.	O
</s>
<s>
Modern	O
hard	B-Device
disk	I-Device
drives	I-Device
appear	O
to	O
their	O
host	B-Architecture
controller	I-Architecture
as	O
a	O
contiguous	O
set	O
of	O
logical	O
blocks	B-General_Concept
,	O
and	O
the	O
gross	O
drive	O
capacity	O
is	O
calculated	O
by	O
multiplying	O
the	O
number	O
of	O
blocks	B-General_Concept
by	O
the	O
block	B-General_Concept
size	I-General_Concept
.	O
</s>
<s>
This	O
information	O
is	O
available	O
from	O
the	O
manufacturer	O
's	O
product	O
specification	O
,	O
and	O
from	O
the	O
drive	O
itself	O
through	O
use	O
of	O
operating	B-General_Concept
system	I-General_Concept
functions	O
that	O
invoke	O
low-level	O
drive	O
commands	O
.	O
</s>
<s>
Older	O
IBM	O
and	O
compatible	O
drives	O
,	O
e.g.	O
,	O
IBM	O
3390	O
,	O
using	O
the	O
CKD	B-Application
record	O
format	O
have	O
variable	O
length	O
records	O
;	O
such	O
drive	O
capacity	O
calculations	O
must	O
take	O
into	O
account	O
the	O
characteristics	O
of	O
the	O
records	O
.	O
</s>
<s>
Some	O
newer	O
DASD	B-Application
simulate	O
CKD	B-Application
,	O
and	O
the	O
same	O
capacity	O
formulae	O
apply	O
.	O
</s>
<s>
The	O
gross	O
capacity	O
of	O
older	O
sector-oriented	O
HDDs	O
is	O
calculated	O
as	O
the	O
product	O
of	O
the	O
number	O
of	O
cylinders	O
per	O
recording	O
zone	O
,	O
the	O
number	O
of	O
bytes	B-Application
per	O
sector	O
(	O
most	O
commonly	O
512	O
)	O
,	O
and	O
the	O
count	O
of	O
zones	B-Architecture
of	O
the	O
drive	O
.	O
</s>
<s>
Some	O
modern	O
SATA	O
drives	O
also	O
report	O
cylinder-head-sector	B-Device
(	O
CHS	O
)	O
capacities	O
,	O
but	O
these	O
are	O
not	O
physical	O
parameters	O
because	O
the	O
reported	O
values	O
are	O
constrained	O
by	O
historic	O
operating	B-General_Concept
system	I-General_Concept
interfaces	B-Application
.	O
</s>
<s>
The	O
C/H/S	B-Device
scheme	O
has	O
been	O
replaced	O
by	O
logical	B-Device
block	I-Device
addressing	I-Device
(	O
LBA	O
)	O
,	O
a	O
simple	O
linear	O
addressing	O
scheme	O
that	O
locates	O
blocks	B-General_Concept
by	O
an	O
integer	O
index	O
,	O
which	O
starts	O
at	O
LBA	O
0	O
for	O
the	O
first	O
block	O
and	O
increments	O
thereafter	O
.	O
</s>
<s>
When	O
using	O
the	O
C/H/S	B-Device
method	O
to	O
describe	O
modern	O
large	O
drives	O
,	O
the	O
number	O
of	O
heads	O
is	O
often	O
set	O
to	O
64	O
,	O
although	O
a	O
typical	O
modern	O
hard	B-Device
disk	I-Device
drive	I-Device
has	O
between	O
one	O
and	O
four	O
platters	B-Device
.	O
</s>
<s>
In	O
modern	O
HDDs	O
,	O
spare	O
capacity	O
for	O
defect	B-Device
management	I-Device
is	O
not	O
included	O
in	O
the	O
published	O
capacity	O
;	O
however	O
,	O
in	O
many	O
early	O
HDDs	O
a	O
certain	O
number	O
of	O
sectors	O
were	O
reserved	O
as	O
spares	O
,	O
thereby	O
reducing	O
the	O
capacity	O
available	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
For	O
RAID	B-Architecture
subsystems	O
,	O
data	O
integrity	O
and	O
fault-tolerance	O
requirements	O
also	O
reduce	O
the	O
realized	O
capacity	O
.	O
</s>
<s>
RAID	B-Architecture
subsystems	O
are	O
multiple	O
drives	O
that	O
appear	O
to	O
be	O
one	O
drive	O
or	O
more	O
drives	O
to	O
the	O
user	O
,	O
but	O
provide	O
fault	O
tolerance	O
.	O
</s>
<s>
Most	O
RAID	B-Architecture
vendors	O
use	O
checksums	B-Algorithm
to	O
improve	O
data	O
integrity	O
at	O
the	O
block	O
level	O
.	O
</s>
<s>
Some	O
vendors	O
design	O
systems	O
using	O
HDDs	O
with	O
sectors	O
of	O
520	O
bytes	B-Application
to	O
contain	O
512	B-Application
bytes	I-Application
of	O
user	O
data	O
and	O
eight	O
checksum	B-Algorithm
bytes	B-Application
,	O
or	O
by	O
using	O
separate	O
512-byte	O
sectors	O
for	O
the	O
checksum	B-Algorithm
data	O
.	O
</s>
<s>
Some	O
systems	O
may	O
use	O
hidden	O
partitions	B-Application
for	O
system	O
recovery	O
,	O
reducing	O
the	O
capacity	O
available	O
to	O
the	O
end	O
user	O
without	O
knowledge	O
of	O
special	B-Application
disk	I-Application
partitioning	I-Application
utilities	I-Application
like	O
diskpart	B-Application
in	O
Windows	O
.	O
</s>
<s>
Data	O
is	O
stored	O
on	O
a	O
hard	B-Device
drive	I-Device
in	O
a	O
series	O
of	O
logical	O
blocks	B-General_Concept
.	O
</s>
<s>
Each	O
block	O
is	O
delimited	O
by	O
markers	O
identifying	O
its	O
start	O
and	O
end	O
,	O
error	O
detecting	O
and	O
correcting	O
information	O
,	O
and	O
space	O
between	O
blocks	B-General_Concept
to	O
allow	O
for	O
minor	O
timing	O
variations	O
.	O
</s>
<s>
These	O
blocks	B-General_Concept
often	O
contained	O
512	B-Application
bytes	I-Application
of	O
usable	O
data	O
,	O
but	O
other	O
sizes	O
have	O
been	O
used	O
.	O
</s>
<s>
As	O
drive	O
density	O
increased	O
,	O
an	O
initiative	O
known	O
as	O
Advanced	B-Device
Format	I-Device
extended	O
the	O
block	B-General_Concept
size	I-General_Concept
to	O
4096	O
bytes	B-Application
of	O
usable	O
data	O
,	O
with	O
a	O
resulting	O
significant	O
reduction	O
in	O
the	O
amount	O
of	O
disk	B-Device
space	O
used	O
for	O
block	O
headers	O
,	O
error	B-Error_Name
checking	I-Error_Name
data	O
,	O
and	O
spacing	O
.	O
</s>
<s>
The	O
process	O
of	O
initializing	O
these	O
logical	O
blocks	B-General_Concept
on	O
the	O
physical	O
disk	B-Device
platters	I-Device
is	O
called	O
low-level	O
formatting	O
,	O
which	O
is	O
usually	O
performed	O
at	O
the	O
factory	O
and	O
is	O
not	O
normally	O
changed	O
in	O
the	O
field	O
.	O
</s>
<s>
High-level	O
formatting	O
writes	O
data	O
structures	O
used	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
organize	O
data	O
files	O
on	O
the	O
disk	B-Device
.	O
</s>
<s>
This	O
includes	O
writing	O
partition	B-Application
and	O
file	B-Application
system	I-Application
structures	O
into	O
selected	O
logical	O
blocks	B-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
some	O
of	O
the	O
disk	B-Device
space	O
will	O
be	O
used	O
to	O
hold	O
a	O
directory	O
of	O
disk	B-Device
file	O
names	O
and	O
a	O
list	O
of	O
logical	O
blocks	B-General_Concept
associated	O
with	O
a	O
particular	O
file	O
.	O
</s>
<s>
Examples	O
of	O
partition	B-Application
mapping	O
scheme	O
include	O
Master	B-Device
boot	I-Device
record	I-Device
(	O
MBR	O
)	O
and	O
GUID	B-Application
Partition	I-Application
Table	I-Application
(	O
GPT	O
)	O
.	O
</s>
<s>
Examples	O
of	O
data	O
structures	O
stored	O
on	O
disk	B-Device
to	O
retrieve	O
files	O
include	O
the	O
File	B-Application
Allocation	I-Application
Table	I-Application
(	O
FAT	O
)	O
in	O
the	O
DOS	B-Application
file	I-Application
system	I-Application
and	O
inodes	B-Application
in	O
many	O
UNIX	B-Application
file	B-Application
systems	I-Application
,	O
as	O
well	O
as	O
other	O
operating	B-General_Concept
system	I-General_Concept
data	O
structures	O
(	O
also	O
known	O
as	O
metadata	O
)	O
.	O
</s>
<s>
+	O
Decimal	O
and	O
binary	O
unit	O
prefixes	O
interpretation	O
Capacity	O
advertised	O
by	O
manufacturers	O
Capacity	O
expected	O
by	O
some	O
consumers	O
Reported	O
capacity	O
Windows	O
macOS	B-Application
ver	O
10.6	B-Device
+	O
With	O
prefix	O
Bytes	B-Application
Bytes	B-Application
Diff	O
.	O
</s>
<s>
Software	O
reports	O
hard	B-Device
disk	I-Device
drive	I-Device
or	O
memory	O
capacity	O
in	O
different	O
forms	O
using	O
either	O
decimal	O
or	O
binary	O
prefixes	O
.	O
</s>
<s>
The	O
Microsoft	B-Application
Windows	I-Application
family	O
of	O
operating	B-General_Concept
systems	I-General_Concept
uses	O
the	O
binary	O
convention	O
when	O
reporting	O
storage	O
capacity	O
,	O
so	O
an	O
HDD	O
offered	O
by	O
its	O
manufacturer	O
as	O
a	O
1TB	O
drive	O
is	O
reported	O
by	O
these	O
operating	B-General_Concept
systems	I-General_Concept
as	O
a	O
931GB	O
HDD	O
.	O
</s>
<s>
Mac	B-Device
OS	I-Device
X	I-Device
10.6	I-Device
(	O
"	O
Snow	B-Device
Leopard	I-Device
"	O
)	O
uses	O
decimal	O
convention	O
when	O
reporting	O
HDD	O
capacity	O
.	O
</s>
<s>
The	O
default	O
behavior	O
of	O
the	O
command-line	B-Device
utility	I-Device
on	O
Linux	O
is	O
to	O
report	O
the	O
HDD	O
capacity	O
as	O
a	O
number	O
of	O
1024-byte	O
units	O
.	O
</s>
<s>
IBM	O
's	O
first	O
hard	B-Device
disk	I-Device
drive	I-Device
,	O
the	O
IBM	O
350	O
,	O
used	O
a	O
stack	O
of	O
fifty	O
24-inch	O
platters	B-Device
,	O
stored	O
3.75	O
MB	O
of	O
data	O
(	O
approximately	O
the	O
size	O
of	O
one	O
modern	O
digital	O
picture	O
)	O
,	O
and	O
was	O
of	O
a	O
size	O
comparable	O
to	O
two	O
large	O
refrigerators	O
.	O
</s>
<s>
In	O
1962	O
,	O
IBM	O
introduced	O
its	O
model	B-Device
1311	I-Device
disk	B-Device
,	O
which	O
used	O
six	O
14-inch	O
(	O
nominal	O
size	O
)	O
platters	B-Device
in	O
a	O
removable	O
pack	O
and	O
was	O
roughly	O
the	O
size	O
of	O
a	O
washing	O
machine	O
.	O
</s>
<s>
The	O
IBM	B-Device
2314	I-Device
used	O
platters	B-Device
of	O
the	O
same	O
size	O
in	O
an	O
eleven-high	O
pack	O
and	O
introduced	O
the	O
"	O
drive	O
in	O
a	O
drawer	O
"	O
layout	O
.	O
</s>
<s>
Beginning	O
in	O
the	O
late	O
1960s	O
drives	O
were	O
offered	O
that	O
fit	O
entirely	O
into	O
a	O
chassis	O
that	O
would	O
mount	O
in	O
a	O
19-inch	B-Application
rack	I-Application
.	O
</s>
<s>
Digital	O
's	O
RK05	B-Device
and	O
RL01	O
were	O
early	O
examples	O
using	O
single	O
14-inch	O
platters	B-Device
in	O
removable	O
packs	O
,	O
the	O
entire	O
drive	O
fitting	O
in	O
a	O
10.5-inch-high	O
rack	O
space	O
(	O
six	O
rack	O
units	O
)	O
.	O
</s>
<s>
In	O
the	O
mid-to-late	O
1980s	O
the	O
similarly	O
sized	O
Fujitsu	B-Device
Eagle	I-Device
,	O
which	O
used	O
(	O
coincidentally	O
)	O
10.5-inch	O
platters	B-Device
,	O
was	O
a	O
popular	O
product	O
.	O
</s>
<s>
With	O
increasing	O
sales	O
of	O
microcomputers	O
having	O
built	O
in	O
floppy-disk	B-Device
drives	I-Device
(	O
FDDs	O
)	O
,	O
HDDs	O
that	O
would	O
fit	O
to	O
the	O
FDD	O
mountings	O
became	O
desirable	O
.	O
</s>
<s>
Starting	O
with	O
the	O
Shugart	O
Associates	O
SA1000	O
,	O
HDD	O
form	O
factors	O
initially	O
followed	O
those	O
of	O
8-inch	O
,	O
5¼-inch	O
,	O
and	O
3½-inch	O
floppy	B-Device
disk	I-Device
drives	I-Device
.	O
</s>
<s>
Because	O
there	O
were	O
no	O
smaller	O
floppy	B-Device
disk	I-Device
drives	I-Device
,	O
smaller	O
HDD	O
form	O
factors	O
such	O
as	O
2½-inch	O
drives	O
(	O
actually	O
2.75	O
"	O
wide	O
)	O
developed	O
from	O
product	O
offerings	O
or	O
industry	O
standards	O
.	O
</s>
<s>
,	O
2½-inch	O
and	O
3½-inch	O
hard	B-Device
disks	I-Device
are	O
the	O
most	O
popular	O
sizes	O
.	O
</s>
<s>
By	O
2009	O
,	O
all	O
manufacturers	O
had	O
discontinued	O
the	O
development	O
of	O
new	O
products	O
for	O
the	O
1.3-inch	O
,	O
1-inch	O
and	O
0.85-inch	O
form	O
factors	O
due	O
to	O
falling	O
prices	O
of	O
flash	B-Device
memory	I-Device
,	O
which	O
has	O
no	O
moving	O
parts	O
.	O
</s>
<s>
The	O
factors	O
that	O
limit	O
the	O
time	B-General_Concept
to	I-General_Concept
access	I-General_Concept
the	I-General_Concept
data	I-General_Concept
on	O
an	O
HDD	O
are	O
mostly	O
related	O
to	O
the	O
mechanical	O
nature	O
of	O
the	O
rotating	O
disks	B-Device
and	O
moving	O
heads	O
,	O
including	O
:	O
</s>
<s>
Seek	O
time	O
is	O
a	O
measure	O
of	O
how	O
long	O
it	O
takes	O
the	O
head	O
assembly	O
to	O
travel	O
to	O
the	O
track	O
of	O
the	O
disk	B-Device
that	O
contains	O
data	O
.	O
</s>
<s>
Rotational	O
latency	O
is	O
incurred	O
because	O
the	O
desired	O
disk	B-Application
sector	I-Application
may	O
not	O
be	O
directly	O
under	O
the	O
head	O
when	O
data	O
transfer	O
is	O
requested	O
.	O
</s>
<s>
The	O
bit	O
rate	O
or	O
data	O
transfer	O
rate	O
(	O
once	O
the	O
head	O
is	O
in	O
the	O
right	O
position	O
)	O
creates	O
delay	O
which	O
is	O
a	O
function	O
of	O
the	O
number	O
of	O
blocks	B-General_Concept
transferred	O
;	O
typically	O
relatively	O
small	O
,	O
but	O
can	O
be	O
quite	O
long	O
with	O
the	O
transfer	O
of	O
large	O
contiguous	O
files	O
.	O
</s>
<s>
Delay	O
may	O
also	O
occur	O
if	O
the	O
drive	O
disks	B-Device
are	O
stopped	O
to	O
save	O
energy	O
.	O
</s>
<s>
Defragmentation	B-Application
is	O
a	O
procedure	O
used	O
to	O
minimize	O
delay	O
in	O
retrieving	O
data	O
by	O
moving	O
related	O
items	O
to	O
physically	O
proximate	O
areas	O
on	O
the	O
disk	B-Device
.	O
</s>
<s>
Some	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
perform	O
defragmentation	B-Application
automatically	O
.	O
</s>
<s>
Although	O
automatic	O
defragmentation	B-Application
is	O
intended	O
to	O
reduce	O
access	O
delays	O
,	O
performance	B-Device
will	O
be	O
temporarily	O
reduced	O
while	O
the	O
procedure	O
is	O
in	O
progress	O
.	O
</s>
<s>
Increasing	O
areal	B-Device
density	I-Device
increases	O
throughput	O
by	O
increasing	O
data	O
rate	O
and	O
by	O
increasing	O
the	O
amount	O
of	O
data	O
under	O
a	O
set	O
of	O
heads	O
,	O
thereby	O
potentially	O
reducing	O
seek	O
activity	O
for	O
a	O
given	O
amount	O
of	O
data	O
.	O
</s>
<s>
The	O
time	O
to	O
access	O
data	O
has	O
not	O
kept	O
up	O
with	O
throughput	O
increases	O
,	O
which	O
themselves	O
have	O
not	O
kept	O
up	O
with	O
growth	O
in	O
bit	B-Device
density	I-Device
and	O
storage	O
capacity	O
.	O
</s>
<s>
,	O
a	O
typical	O
7,200	O
-rpm	O
desktop	B-Device
HDD	O
has	O
a	O
sustained	O
"	O
disk-to-buffer	O
"	O
data	O
transfer	O
rate	O
up	O
to	O
1,030	O
Mbit/s	O
.	O
</s>
<s>
A	O
current	O
widely	O
used	O
standard	O
for	O
the	O
"	O
buffer-to-computer	O
"	O
interface	B-Application
is	O
3.0Gbit/s	O
SATA	O
,	O
which	O
can	O
send	O
about	O
300	O
megabyte/s	O
(	O
10-bit	O
encoding	O
)	O
from	O
the	O
buffer	B-Device
to	O
the	O
computer	O
,	O
and	O
thus	O
is	O
still	O
comfortably	O
ahead	O
of	O
today	O
's	O
disk-to-buffer	O
transfer	O
rates	O
.	O
</s>
<s>
Data	O
transfer	O
rate	O
(	O
read/write	O
)	O
can	O
be	O
measured	O
by	O
writing	O
a	O
large	O
file	O
to	O
disk	B-Device
using	O
special	O
file	O
generator	O
tools	O
,	O
then	O
reading	O
back	O
the	O
file	O
.	O
</s>
<s>
Transfer	O
rate	O
can	O
be	O
influenced	O
by	O
file	B-Application
system	I-Application
fragmentation	I-Application
and	O
the	O
layout	O
of	O
the	O
files	O
.	O
</s>
<s>
HDD	O
data	O
transfer	O
rate	O
depends	O
upon	O
the	O
rotational	O
speed	O
of	O
the	O
platters	B-Device
and	O
the	O
data	B-General_Concept
recording	I-General_Concept
density	O
.	O
</s>
<s>
While	O
areal	B-Device
density	I-Device
advances	O
by	O
increasing	O
both	O
the	O
number	O
of	O
tracks	O
across	O
the	O
disk	B-Device
and	O
the	O
number	O
of	O
sectors	O
per	O
track	O
,	O
only	O
the	O
latter	O
increases	O
the	O
data	O
transfer	O
rate	O
for	O
a	O
given	O
rpm	O
.	O
</s>
<s>
Since	O
data	O
transfer	O
rate	O
performance	B-Device
tracks	O
only	O
one	O
of	O
the	O
two	O
components	O
of	O
areal	B-Device
density	I-Device
,	O
its	O
performance	B-Device
improves	O
at	O
a	O
lower	O
rate	O
.	O
</s>
<s>
Other	O
performance	B-Device
considerations	O
include	O
quality-adjusted	O
price	O
,	O
power	O
consumption	O
,	O
audible	O
noise	O
,	O
and	O
both	O
operating	O
and	O
non-operating	O
shock	O
resistance	O
.	O
</s>
<s>
Current	O
hard	B-Device
drives	I-Device
connect	O
to	O
a	O
computer	O
over	O
one	O
of	O
several	O
bus	B-General_Concept
types	O
,	O
including	O
parallel	B-Protocol
ATA	I-Protocol
,	O
Serial	O
ATA	B-Protocol
,	O
SCSI	B-Architecture
,	O
Serial	O
Attached	O
SCSI	B-Architecture
(	O
SAS	O
)	O
,	O
and	O
Fibre	B-Architecture
Channel	I-Architecture
.	O
</s>
<s>
Some	O
drives	O
,	O
especially	O
external	O
portable	O
drives	O
,	O
use	O
IEEE	O
1394	O
,	O
or	O
USB	B-Protocol
.	O
</s>
<s>
All	O
of	O
these	O
interfaces	B-Application
are	O
digital	O
;	O
electronics	O
on	O
the	O
drive	O
process	O
the	O
analog	O
signals	O
from	O
the	O
read/write	B-Device
heads	I-Device
.	O
</s>
<s>
Current	O
drives	O
present	O
a	O
consistent	O
interface	B-Application
to	O
the	O
rest	O
of	O
the	O
computer	O
,	O
independent	O
of	O
the	O
data	O
encoding	O
scheme	O
used	O
internally	O
,	O
and	O
independent	O
of	O
the	O
physical	O
number	O
of	O
disks	B-Device
and	O
heads	O
within	O
the	O
drive	O
.	O
</s>
<s>
Typically	O
a	O
DSP	B-Architecture
in	O
the	O
electronics	O
inside	O
the	O
drive	O
takes	O
the	O
raw	O
analog	O
voltages	O
from	O
the	O
read	O
head	O
and	O
uses	O
PRML	B-Device
and	O
Reed	B-Error_Name
–	I-Error_Name
Solomon	I-Error_Name
error	I-Error_Name
correction	I-Error_Name
to	O
decode	O
the	O
data	O
,	O
then	O
sends	O
that	O
data	O
out	O
the	O
standard	O
interface	B-Application
.	O
</s>
<s>
That	O
DSP	B-Architecture
also	O
watches	O
the	O
error	O
rate	O
detected	O
by	O
error	B-Error_Name
detection	I-Error_Name
and	I-Error_Name
correction	I-Error_Name
,	O
and	O
performs	O
bad	B-Device
sector	I-Device
remapping	O
,	O
data	O
collection	O
for	O
Self-Monitoring	O
,	O
Analysis	O
,	O
and	O
Reporting	O
Technology	O
,	O
and	O
other	O
internal	O
tasks	O
.	O
</s>
<s>
Modern	O
interfaces	B-Application
connect	O
the	O
drive	O
to	O
the	O
host	O
interface	B-Application
with	O
a	O
single	O
data/control	O
cable	O
.	O
</s>
<s>
Older	O
interfaces	B-Application
had	O
separate	O
cables	O
for	O
data	O
signals	O
and	O
for	O
drive	O
control	O
signals	O
.	O
</s>
<s>
Small	B-Architecture
Computer	I-Architecture
System	I-Architecture
Interface	I-Architecture
(	O
SCSI	B-Architecture
)	O
,	O
originally	O
named	O
SASI	O
for	O
Shugart	O
Associates	O
System	O
Interface	B-Application
,	O
was	O
standard	O
on	O
servers	B-Application
,	O
workstations	O
,	O
Commodore	O
Amiga	O
,	O
Atari	B-Device
ST	I-Device
and	O
Apple	B-Device
Macintosh	I-Device
computers	I-Device
through	O
the	O
mid-1990s	O
,	O
by	O
which	O
time	O
most	O
models	O
had	O
been	O
transitioned	O
to	O
newer	O
interfaces	B-Application
.	O
</s>
<s>
The	O
length	O
limit	O
of	O
the	O
data	O
cable	O
allows	O
for	O
external	O
SCSI	B-Architecture
devices	O
.	O
</s>
<s>
The	O
SCSI	B-Architecture
command	O
set	O
is	O
still	O
used	O
in	O
the	O
more	O
modern	O
SAS	O
interface	B-Application
.	O
</s>
<s>
Integrated	B-Protocol
Drive	I-Protocol
Electronics	I-Protocol
(	O
IDE	O
)	O
,	O
later	O
standardized	O
under	O
the	O
name	O
AT	B-Protocol
Attachment	I-Protocol
(	O
ATA	B-Protocol
,	O
with	O
the	O
alias	O
PATA	B-Protocol
(	O
Parallel	B-Protocol
ATA	I-Protocol
)	O
retroactively	O
added	O
upon	O
introduction	O
of	O
SATA	O
)	O
moved	O
the	O
HDD	O
controller	O
from	O
the	O
interface	B-Application
card	O
to	O
the	O
disk	B-Device
drive	I-Device
.	O
</s>
<s>
This	O
helped	O
to	O
standardize	O
the	O
host/controller	O
interface	B-Application
,	O
reduce	O
the	O
programming	O
complexity	O
in	O
the	O
host	O
device	O
driver	O
,	O
and	O
reduced	O
system	O
cost	O
and	O
complexity	O
.	O
</s>
<s>
The	O
40-pin	O
IDE/ATA	O
connection	O
transfers	O
16bits	O
of	O
data	O
at	O
a	O
time	O
on	O
the	O
data	O
cable	O
.	O
</s>
<s>
EIDE	O
was	O
an	O
unofficial	O
update	O
(	O
by	O
Western	O
Digital	O
)	O
to	O
the	O
original	O
IDE	O
standard	O
,	O
with	O
the	O
key	O
improvement	O
being	O
the	O
use	O
of	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
(	O
DMA	O
)	O
to	O
transfer	O
data	O
between	O
the	O
disk	B-Device
and	O
the	O
computer	O
without	O
the	O
involvement	O
of	O
the	O
CPU	B-General_Concept
,	O
an	O
improvement	O
later	O
adopted	O
by	O
the	O
official	O
ATA	B-Protocol
standards	O
.	O
</s>
<s>
By	O
directly	O
transferring	O
data	O
between	O
memory	O
and	O
disk	B-Device
,	O
DMA	O
eliminates	O
the	O
need	O
for	O
the	O
CPU	B-General_Concept
to	O
copy	O
byte	B-Application
per	O
byte	B-Application
,	O
therefore	O
allowing	O
it	O
to	O
process	O
other	O
tasks	O
while	O
the	O
data	O
transfer	O
occurs	O
.	O
</s>
<s>
Fibre	B-Architecture
Channel	I-Architecture
(	O
FC	O
)	O
is	O
a	O
successor	O
to	O
parallel	O
SCSI	B-Architecture
interface	B-Application
on	O
enterprise	O
market	O
.	O
</s>
<s>
In	O
disk	B-General_Concept
drives	I-General_Concept
usually	O
the	O
Fibre	B-Architecture
Channel	I-Architecture
Arbitrated	I-Architecture
Loop	I-Architecture
(	O
FC-AL	B-Architecture
)	O
connection	O
topology	O
is	O
used	O
.	O
</s>
<s>
FC	O
has	O
much	O
broader	O
usage	O
than	O
mere	O
disk	B-Device
interfaces	B-Application
,	O
and	O
it	O
is	O
the	O
cornerstone	O
of	O
storage	O
area	O
networks	O
(	O
SANs	O
)	O
.	O
</s>
<s>
Recently	O
other	O
protocols	O
for	O
this	O
field	O
,	O
like	O
iSCSI	B-Protocol
and	O
ATA	B-Protocol
over	I-Protocol
Ethernet	I-Protocol
have	O
been	O
developed	O
as	O
well	O
.	O
</s>
<s>
Confusingly	O
,	O
drives	O
usually	O
use	O
copper	O
twisted-pair	O
cables	O
for	O
Fibre	B-Architecture
Channel	I-Architecture
,	O
not	O
fibre	O
optics	O
.	O
</s>
<s>
The	O
latter	O
are	O
traditionally	O
reserved	O
for	O
larger	O
devices	O
,	O
such	O
as	O
servers	B-Application
or	O
disk	B-Device
array	I-Device
controllers	I-Device
.	O
</s>
<s>
Serial	O
Attached	O
SCSI	B-Architecture
(	O
SAS	O
)	O
.	O
</s>
<s>
SAS	O
uses	O
a	O
mechanically	O
compatible	O
data	O
and	O
power	O
connector	O
to	O
standard	O
3.5-inch	O
SATA1/SATA2	O
HDDs	O
,	O
and	O
many	O
server-oriented	O
SAS	O
RAID	B-Device
controllers	I-Device
are	O
also	O
capable	O
of	O
addressing	O
SATA	O
HDDs	O
.	O
</s>
<s>
SAS	O
uses	O
serial	O
communication	O
instead	O
of	O
the	O
parallel	O
method	O
found	O
in	O
traditional	O
SCSI	B-Architecture
devices	O
but	O
still	O
uses	O
SCSI	B-Architecture
commands	O
.	O
</s>
<s>
Serial	O
ATA	B-Protocol
(	O
SATA	O
)	O
.	O
</s>
<s>
A	O
similar	O
differential	O
signaling	O
system	O
is	O
used	O
in	O
RS485	O
,	O
LocalTalk	B-Device
,	O
USB	B-Protocol
,	O
FireWire	B-Protocol
,	O
and	O
differential	O
SCSI	B-Architecture
.	O
</s>
<s>
SATA	O
I	O
to	O
III	O
are	O
designed	O
to	O
be	O
compatible	O
with	O
,	O
and	O
use	O
,	O
a	O
subset	O
of	O
SAS	O
commands	O
,	O
and	O
compatible	O
interfaces	B-Application
.	O
</s>
<s>
Therefore	O
,	O
a	O
SATA	O
hard	B-Device
drive	I-Device
can	O
be	O
connected	O
to	O
and	O
controlled	O
by	O
a	O
SAS	O
hard	B-General_Concept
drive	I-General_Concept
controller	I-General_Concept
(	O
with	O
some	O
minor	O
exceptions	O
such	O
as	O
drives/controllers	O
with	O
limited	O
compatibility	O
)	O
.	O
</s>
<s>
Due	O
to	O
the	O
extremely	O
close	O
spacing	O
between	O
the	O
heads	O
and	O
the	O
disk	B-Device
surface	O
,	O
HDDs	O
are	O
vulnerable	O
to	O
being	O
damaged	O
by	O
a	O
head	O
crash	O
–	O
a	O
failure	B-Device
of	I-Device
the	I-Device
disk	I-Device
in	O
which	O
the	O
head	O
scrapes	O
across	O
the	O
platter	O
surface	O
,	O
often	O
grinding	O
away	O
the	O
thin	O
magnetic	O
film	O
and	O
causing	O
data	O
loss	O
.	O
</s>
<s>
Head	O
crashes	O
can	O
be	O
caused	O
by	O
electronic	O
failure	O
,	O
a	O
sudden	O
power	O
failure	O
,	O
physical	O
shock	O
,	O
contamination	O
of	O
the	O
drive	O
's	O
internal	O
enclosure	B-Device
,	O
wear	O
and	O
tear	O
,	O
corrosion	O
,	O
or	O
poorly	O
manufactured	O
platters	B-Device
and	O
heads	O
.	O
</s>
<s>
The	O
HDD	O
's	O
spindle	O
system	O
relies	O
on	O
air	O
density	O
inside	O
the	O
disk	B-Device
enclosure	I-Device
to	O
support	O
the	O
heads	O
at	O
their	O
proper	O
flying	B-Device
height	I-Device
while	O
the	O
disk	B-Device
rotates	O
.	O
</s>
<s>
The	O
connection	O
to	O
the	O
external	O
environment	O
and	O
density	O
occurs	O
through	O
a	O
small	O
hole	O
in	O
the	O
enclosure	B-Device
(	O
about	O
0.5mm	O
in	O
breadth	O
)	O
,	O
usually	O
with	O
a	O
filter	O
on	O
the	O
inside	O
(	O
the	O
breather	O
filter	O
)	O
.	O
</s>
<s>
If	O
the	O
air	O
density	O
is	O
too	O
low	O
,	O
then	O
there	O
is	O
not	O
enough	O
lift	O
for	O
the	O
flying	O
head	O
,	O
so	O
the	O
head	O
gets	O
too	O
close	O
to	O
the	O
disk	B-Device
,	O
and	O
there	O
is	O
a	O
risk	O
of	O
head	O
crashes	O
and	O
data	O
loss	O
.	O
</s>
<s>
Specially	O
manufactured	O
sealed	O
and	O
pressurized	O
disks	B-Device
are	O
needed	O
for	O
reliable	O
high-altitude	O
operation	O
,	O
above	O
about	O
.	O
</s>
<s>
Modern	O
disks	B-Device
include	O
temperature	O
sensors	O
and	O
adjust	O
their	O
operation	O
to	O
the	O
operating	O
environment	O
.	O
</s>
<s>
Breather	O
holes	O
can	O
be	O
seen	O
on	O
all	O
disk	B-General_Concept
drives	I-General_Concept
–	O
they	O
usually	O
have	O
a	O
sticker	O
next	O
to	O
them	O
,	O
warning	O
the	O
user	O
not	O
to	O
cover	O
the	O
holes	O
.	O
</s>
<s>
The	O
air	O
inside	O
the	O
operating	O
drive	O
is	O
constantly	O
moving	O
too	O
,	O
being	O
swept	O
in	O
motion	O
by	O
friction	O
with	O
the	O
spinning	O
platters	B-Device
.	O
</s>
<s>
This	O
air	O
passes	O
through	O
an	O
internal	O
recirculation	O
(	O
or	O
"	O
recirc	O
"	O
)	O
filter	O
to	O
remove	O
any	O
leftover	O
contaminants	O
from	O
manufacture	O
,	O
any	O
particles	O
or	O
chemicals	O
that	O
may	O
have	O
somehow	O
entered	O
the	O
enclosure	B-Device
,	O
and	O
any	O
particles	O
or	O
outgassing	O
generated	O
internally	O
in	O
normal	O
operation	O
.	O
</s>
<s>
Very	O
high	O
humidity	O
present	O
for	O
extended	O
periods	O
of	O
time	O
can	O
corrode	O
the	O
heads	O
and	O
platters	B-Device
.	O
</s>
<s>
For	O
giant	B-General_Concept
magnetoresistive	I-General_Concept
(	O
GMR	B-General_Concept
)	O
heads	O
in	O
particular	O
,	O
a	O
minor	O
head	O
crash	O
from	O
contamination	O
(	O
that	O
does	O
not	O
remove	O
the	O
magnetic	O
surface	O
of	O
the	O
disk	B-Device
)	O
still	O
results	O
in	O
the	O
head	O
temporarily	O
overheating	O
,	O
due	O
to	O
friction	O
with	O
the	O
disk	B-Device
surface	O
,	O
and	O
can	O
render	O
the	O
data	O
unreadable	O
for	O
a	O
short	O
period	O
until	O
the	O
head	O
temperature	O
stabilizes	O
(	O
so	O
called	O
"	O
thermal	O
asperity	O
"	O
,	O
a	O
problem	O
which	O
can	O
partially	O
be	O
dealt	O
with	O
by	O
proper	O
electronic	O
filtering	O
of	O
the	O
read	O
signal	O
)	O
.	O
</s>
<s>
When	O
the	O
logic	O
board	O
of	O
a	O
hard	B-Device
disk	I-Device
fails	O
,	O
the	O
drive	O
can	O
often	O
be	O
restored	O
to	O
functioning	O
order	O
and	O
the	O
data	O
recovered	O
by	O
replacing	O
the	O
circuit	O
board	O
with	O
one	O
of	O
an	O
identical	O
hard	B-Device
disk	I-Device
.	O
</s>
<s>
In	O
the	O
case	O
of	O
read-write	B-Device
head	I-Device
faults	O
,	O
they	O
can	O
be	O
replaced	O
using	O
specialized	O
tools	O
in	O
a	O
dust-free	O
environment	O
.	O
</s>
<s>
If	O
the	O
disk	B-Device
platters	I-Device
are	O
undamaged	O
,	O
they	O
can	O
be	O
transferred	O
into	O
an	O
identical	O
enclosure	B-Device
and	O
the	O
data	O
can	O
be	O
copied	O
or	O
cloned	O
onto	O
a	O
new	O
drive	O
.	O
</s>
<s>
In	O
the	O
event	O
of	O
disk-platter	O
failures	O
,	O
disassembly	O
and	O
imaging	O
of	O
the	O
disk	B-Device
platters	I-Device
may	O
be	O
required	O
.	O
</s>
<s>
For	O
logical	O
damage	O
to	O
file	B-Application
systems	I-Application
,	O
a	O
variety	O
of	O
tools	O
,	O
including	O
fsck	B-Device
on	O
UNIX-like	B-Operating_System
systems	I-Operating_System
and	O
CHKDSK	B-Device
on	O
Windows	O
,	O
can	O
be	O
used	O
for	O
data	B-Application
recovery	I-Application
.	O
</s>
<s>
A	O
common	O
expectation	O
is	O
that	O
hard	B-Device
disk	I-Device
drives	I-Device
designed	O
and	O
marketed	O
for	O
server	O
use	O
will	O
fail	O
less	O
frequently	O
than	O
consumer-grade	O
drives	O
usually	O
used	O
in	O
desktop	B-Device
computers	I-Device
.	O
</s>
<s>
However	O
,	O
two	O
independent	O
studies	O
by	O
Carnegie	O
Mellon	O
University	O
and	O
Google	B-Application
found	O
that	O
the	O
"	O
grade	O
"	O
of	O
a	O
drive	O
does	O
not	O
relate	O
to	O
the	O
drive	O
's	O
failure	O
rate	O
.	O
</s>
<s>
A	O
2011	O
summary	O
of	O
research	O
,	O
into	O
SSD	B-Device
and	O
magnetic	B-Architecture
disk	I-Architecture
failure	O
patterns	O
by	O
Tom	O
's	O
Hardware	O
summarized	O
research	O
findings	O
as	O
follows	O
:	O
</s>
<s>
Backblaze	O
subsequently	O
reported	O
that	O
the	O
failure	O
rate	O
for	O
HDDs	O
and	O
SSD	B-Device
of	O
equivalent	O
age	O
was	O
similar	O
.	O
</s>
<s>
Desktop	B-Device
HDDs	O
typically	O
have	O
two	O
to	O
five	O
internal	O
platters	B-Device
,	O
rotate	O
at	O
5,400	O
to	O
10,000	O
rpm	O
,	O
and	O
have	O
a	O
media	O
transfer	O
rate	O
of	O
0.5Gbit/s	O
or	O
higher	O
(	O
1GB	O
=	O
109	O
bytes	B-Application
;	O
1Gbit/s	O
=	O
109	O
bit/s	O
)	O
.	O
</s>
<s>
,	O
the	O
highest-capacity	O
desktop	B-Device
HDDs	O
stored	O
16TB	O
,	O
with	O
plans	O
to	O
release	O
18	O
TB	O
drives	O
later	O
in	O
2019	O
.	O
</s>
<s>
18	O
TB	O
HDDs	O
were	O
released	O
in	O
2020	O
.	O
,	O
the	O
typical	O
speed	O
of	O
a	O
hard	B-Device
drive	I-Device
in	O
an	O
average	O
desktop	B-Device
computer	I-Device
is	O
7,200	O
RPM	O
,	O
whereas	O
low-cost	O
desktop	B-Device
computers	I-Device
may	O
use	O
5,900	O
RPM	O
or	O
5,400	O
RPM	O
drives	O
.	O
</s>
<s>
For	O
some	O
time	O
in	O
the	O
2000s	O
and	O
early	O
2010s	O
some	O
desktop	B-Device
users	O
and	O
data	O
centers	O
also	O
used	O
10,000	O
RPM	O
drives	O
such	O
as	O
Western	B-Device
Digital	I-Device
Raptor	I-Device
but	O
such	O
drives	O
have	O
become	O
much	O
rarer	O
and	O
are	O
not	O
commonly	O
used	O
now	O
,	O
having	O
been	O
replaced	O
by	O
NAND	O
flash-based	O
SSDs	B-Device
.	O
</s>
<s>
Smaller	O
than	O
their	O
desktop	B-Device
and	O
enterprise	O
counterparts	O
,	O
they	O
tend	O
to	O
be	O
slower	O
and	O
have	O
lower	O
capacity	O
,	O
because	O
typically	O
has	O
one	O
internal	O
platter	O
and	O
were	O
2.5	O
"	O
or	O
1.8	O
"	O
physical	O
size	O
instead	O
of	O
more	O
common	O
for	O
desktops	O
3.5	B-Device
"	I-Device
form-factor	O
.	O
</s>
<s>
Because	O
of	O
smaller	O
platter(s )	O
,	O
mobile	O
HDDs	O
generally	O
have	O
lower	O
capacity	O
than	O
their	O
desktop	B-Device
counterparts	O
.	O
</s>
<s>
They	O
include	O
drives	O
embedded	O
into	O
digital	B-Application
video	I-Application
recorders	I-Application
and	O
automotive	O
vehicles	O
.	O
</s>
<s>
Current	O
external	O
hard	B-Device
disk	I-Device
drives	I-Device
typically	O
connect	O
via	O
USB-C	O
;	O
earlier	O
models	O
use	O
an	O
regular	O
USB	B-Protocol
(	O
sometimes	O
with	O
using	O
of	O
a	O
pair	O
of	O
ports	O
for	O
better	O
bandwidth	O
)	O
or	O
(	O
rarely	O
)	O
,	O
e.g.	O
,	O
eSATA	O
connection	O
.	O
</s>
<s>
Variants	O
using	O
USB2.0	O
interface	B-Application
generally	O
have	O
slower	O
data	O
transfer	O
rates	O
when	O
compared	O
to	O
internally	O
mounted	O
hard	B-Device
drives	I-Device
connected	O
through	O
SATA	O
.	O
</s>
<s>
Plug	B-Device
and	I-Device
play	I-Device
drive	O
functionality	O
offers	O
system	O
compatibility	O
and	O
features	O
large	O
storage	O
options	O
and	O
portable	O
design	O
.	O
</s>
<s>
,	O
available	O
capacities	O
for	O
external	O
hard	B-Device
disk	I-Device
drives	I-Device
ranged	O
from	O
500GB	O
to	O
10TB	O
.	O
</s>
<s>
External	O
hard	B-Device
disk	I-Device
drives	I-Device
are	O
usually	O
available	O
as	O
assembled	O
integrated	O
products	O
but	O
may	O
be	O
also	O
assembled	O
by	O
combining	O
an	O
external	O
enclosure	B-Device
(	O
with	O
USB	B-Protocol
or	O
other	O
interface	B-Application
)	O
with	O
a	O
separately	O
purchased	O
drive	O
.	O
</s>
<s>
They	O
are	O
available	O
in	O
2.5-inch	O
and	O
3.5-inch	O
sizes	O
;	O
2.5-inch	O
variants	O
are	O
typically	O
called	O
portable	O
external	B-Device
drives	I-Device
,	O
while	O
3.5-inch	O
variants	O
are	O
referred	O
to	O
as	O
desktop	B-Device
external	B-Device
drives	I-Device
.	O
</s>
<s>
"	O
Portable	O
"	O
drives	O
are	O
packaged	O
in	O
smaller	O
and	O
lighter	O
enclosures	O
than	O
the	O
"	O
desktop	B-Device
"	O
drives	O
;	O
additionally	O
,	O
"	O
portable	O
"	O
drives	O
use	O
power	O
provided	O
by	O
the	O
USB	B-Protocol
connection	O
,	O
while	O
"	O
desktop	B-Device
"	O
drives	O
require	O
external	O
power	O
bricks	O
.	O
</s>
<s>
Features	O
such	O
as	O
encryption	O
,	O
Wi-Fi	O
connectivity	O
,	O
biometric	O
security	O
or	O
multiple	O
interfaces	B-Application
(	O
for	O
example	O
,	O
FireWire	B-Protocol
)	O
are	O
available	O
at	O
a	O
higher	O
cost	O
.	O
</s>
<s>
There	O
are	O
pre-assembled	O
external	O
hard	B-Device
disk	I-Device
drives	I-Device
that	O
,	O
when	O
taken	O
out	O
from	O
their	O
enclosures	O
,	O
cannot	O
be	O
used	O
internally	O
in	O
a	O
laptop	O
or	O
desktop	B-Device
computer	I-Device
due	O
to	O
embedded	O
USB	B-Protocol
interface	B-Application
on	O
their	O
printed	O
circuit	O
boards	O
,	O
and	O
lack	O
of	O
SATA	O
(	O
or	O
Parallel	B-Protocol
ATA	I-Protocol
)	O
interfaces	B-Application
.	O
</s>
<s>
Typically	O
used	O
with	O
multiple-user	O
computers	O
running	O
enterprise	B-Application
software	I-Application
.	O
</s>
<s>
Enterprise	O
drives	O
commonly	O
operate	O
continuously	O
(	O
"	O
24/7	O
"	O
)	O
in	O
demanding	O
environments	O
while	O
delivering	O
the	O
highest	O
possible	O
performance	B-Device
without	O
sacrificing	O
reliability	O
.	O
</s>
<s>
Drives	O
running	O
at	O
10,000	O
or	O
15,000	O
rpm	O
use	O
smaller	O
platters	B-Device
to	O
mitigate	O
increased	O
power	O
requirements	O
(	O
as	O
they	O
have	O
less	O
air	O
drag	O
)	O
and	O
therefore	O
generally	O
have	O
lower	O
capacity	O
than	O
the	O
highest	O
capacity	O
desktop	B-Device
drives	O
.	O
</s>
<s>
Enterprise	O
HDDs	O
are	O
commonly	O
connected	O
through	O
Serial	O
Attached	O
SCSI	B-Architecture
(	O
SAS	O
)	O
or	O
Fibre	B-Architecture
Channel	I-Architecture
(	O
FC	O
)	O
.	O
</s>
<s>
Some	O
support	O
multiple	O
ports	O
,	O
so	O
they	O
can	O
be	O
connected	O
to	O
a	O
redundant	O
host	B-Architecture
bus	I-Architecture
adapter	I-Architecture
.	O
</s>
<s>
Enterprise	O
HDDs	O
can	O
have	O
sector	O
sizes	O
larger	O
than	O
512	B-Application
bytes	I-Application
(	O
often	O
520	O
,	O
524	O
,	O
528	O
or	O
536	O
bytes	B-Application
)	O
.	O
</s>
<s>
The	O
additional	O
per-sector	O
space	O
can	O
be	O
used	O
by	O
hardware	O
RAID	B-Device
controllers	I-Device
or	O
applications	O
for	O
storing	O
Data	B-Error_Name
Integrity	I-Error_Name
Field	I-Error_Name
(	O
DIF	O
)	O
or	O
Data	B-Error_Name
Integrity	I-Error_Name
Extensions	I-Error_Name
(	O
DIX	O
)	O
data	O
,	O
resulting	O
in	O
higher	O
reliability	O
and	O
prevention	O
of	O
silent	O
data	O
corruption	O
.	O
</s>
<s>
HDD	O
price	O
per	O
byte	B-Application
decreased	O
at	O
the	O
rate	O
of	O
40%	O
per	O
year	O
during	O
1988	O
–	O
1996	O
,	O
51%	O
per	O
year	O
during	O
1996	O
–	O
2003	O
and	O
34%	O
per	O
year	O
during	O
2003	O
–	O
2010	O
.	O
</s>
<s>
The	O
price	O
decrease	O
slowed	O
down	O
to	O
13%	O
per	O
year	O
during	O
2011	O
–	O
2014	O
,	O
as	O
areal	B-Device
density	I-Device
increase	O
slowed	O
and	O
the	O
2011	O
Thailand	O
floods	O
damaged	O
manufacturing	O
facilities	O
and	O
have	O
held	O
at	O
11%	O
per	O
year	O
during	O
2010	O
–	O
2017	O
.	O
</s>
<s>
The	O
Federal	O
Reserve	O
Board	O
has	O
published	O
a	O
quality-adjusted	O
price	O
index	O
for	O
large-scale	O
enterprise	B-General_Concept
storage	I-General_Concept
systems	O
including	O
three	O
or	O
more	O
enterprise	O
HDDs	O
and	O
associated	O
controllers	O
,	O
racks	O
and	O
cables	O
.	O
</s>
<s>
Worldwide	O
revenue	O
for	O
disk	B-Device
storage	I-Device
declined	O
eight	O
percent	O
per	O
year	O
,	O
from	O
a	O
peak	O
of	O
$38	O
billion	O
in	O
2012	O
to	O
$22	O
billion	O
(	O
estimated	O
)	O
in	O
2019	O
.	O
</s>
<s>
HDDs	O
are	O
being	O
superseded	O
by	O
solid-state	B-Device
drives	I-Device
(	O
SSDs	B-Device
)	O
in	O
markets	O
where	O
their	O
higher	O
speed	O
(	O
up	O
to	O
7	O
gigabytes	O
)	O
per	O
second	O
for	O
M.2	B-Protocol
(	O
NGFF	B-Protocol
)	O
NVMe	B-Application
SSDs	B-Device
,	O
or	O
2.5	O
gigabytes	O
per	O
second	O
for	O
PCIe	O
expansion	O
card	O
drives	O
)	O
,	O
ruggedness	O
,	O
and	O
lower	O
power	O
are	O
more	O
important	O
than	O
price	O
,	O
since	O
the	O
bit	O
cost	O
of	O
SSDs	B-Device
is	O
four	O
to	O
nine	O
times	O
higher	O
than	O
HDDs	O
.	O
</s>
<s>
,	O
HDDs	O
are	O
reported	O
to	O
have	O
a	O
failure	O
rate	O
of	O
2	O
–	O
9%	O
per	O
year	O
,	O
while	O
SSDs	B-Device
have	O
fewer	O
failures	O
:	O
1	O
–	O
3%	O
per	O
year	O
.	O
</s>
<s>
However	O
,	O
SSDs	B-Device
have	O
more	O
un-correctable	O
data	O
errors	O
than	O
HDDs	O
.	O
</s>
<s>
SSDs	B-Device
offer	O
larger	O
capacities	O
(	O
up	O
to	O
100TB	O
)	O
than	O
the	O
largest	O
HDD	O
and/or	O
higher	O
storage	B-Device
densities	I-Device
(	O
100TB	O
and	O
30TB	O
SSDs	B-Device
are	O
housed	O
in	O
2.5	O
inch	O
HDD	O
cases	O
but	O
with	O
the	O
same	O
height	O
as	O
a	O
3.5-inch	O
HDD	O
)	O
,	O
although	O
their	O
cost	O
remains	O
prohibitive	O
.	O
</s>
<s>
A	O
laboratory	O
demonstration	O
of	O
a	O
1.33-Tb	O
3D	O
NAND	O
chip	O
with	O
96	O
layers	O
(	O
NAND	O
commonly	O
used	O
in	O
solid	B-Device
state	I-Device
drives	I-Device
(	O
SSDs	B-Device
)	O
)	O
had	O
5.5Tbit/in2	O
,	O
while	O
the	O
maximum	O
areal	B-Device
density	I-Device
for	O
HDDs	O
is	O
1.5Tbit/in2	O
.	O
</s>
<s>
The	O
areal	B-Device
density	I-Device
of	O
flash	B-Device
memory	I-Device
is	O
doubling	O
every	O
two	O
years	O
,	O
similar	O
to	O
Moore	O
's	O
law	O
(	O
40%	O
per	O
year	O
)	O
and	O
faster	O
than	O
the	O
10	O
–	O
20%	O
per	O
year	O
for	O
HDDs	O
.	O
</s>
<s>
,	O
the	O
maximum	O
capacity	O
was	O
16terabytes	O
for	O
an	O
HDD	O
,	O
and	O
100terabytes	O
for	O
an	O
SSD	B-Device
.	O
</s>
<s>
HDDs	O
were	O
used	O
in	O
70%	O
of	O
the	O
desktop	B-Device
and	O
notebook	O
computers	O
produced	O
in	O
2016	O
,	O
and	O
SSDs	B-Device
were	O
used	O
in	O
30%	O
.	O
</s>
<s>
The	O
usage	O
share	O
of	O
HDDs	O
is	O
declining	O
and	O
could	O
drop	O
below	O
50%	O
in	O
2018	O
–	O
2019	O
according	O
to	O
one	O
forecast	O
,	O
because	O
SSDs	B-Device
are	O
replacing	O
smaller-capacity	O
(	O
less	O
than	O
one-terabyte	O
)	O
HDDs	O
in	O
desktop	B-Device
and	O
notebook	O
computers	O
and	O
MP3	O
players	O
.	O
</s>
<s>
The	O
market	O
for	O
silicon-based	O
flash	B-Device
memory	I-Device
(	O
NAND	O
)	O
chips	O
,	O
used	O
in	O
SSDs	B-Device
and	O
other	O
applications	O
,	O
is	O
growing	O
faster	O
than	O
for	O
HDDs	O
.	O
</s>
