<s>
The	O
Macintosh	B-Device
Hard	B-Device
Disk	I-Device
20	I-Device
was	O
the	O
first	O
hard	B-Device
drive	I-Device
developed	O
by	O
Apple	O
Computer	O
specifically	O
for	O
use	O
with	O
the	O
Macintosh	B-Device
512K	I-Device
.	O
</s>
<s>
Introduced	O
on	O
September	O
17	O
,	O
1985	O
,	O
it	O
was	O
part	O
of	O
Apple	O
's	O
long-awaited	O
solution	O
toward	O
completing	O
the	O
Macintosh	B-Device
Office	I-Device
(	O
a	O
suite	O
of	O
integrated	O
business	O
hardware	O
&	O
software	O
)	O
announced	O
in	O
January	O
1985	O
.	O
</s>
<s>
It	O
would	O
be	O
over	O
a	O
year	O
more	O
before	O
Apple	O
would	O
release	O
the	O
file	O
server	O
software	O
AppleShare	B-General_Concept
that	O
would	O
link	O
all	O
of	O
the	O
hardware	O
together	O
.	O
</s>
<s>
By	O
that	O
time	O
the	O
SCSI	B-Architecture
interface	O
introduced	O
on	O
the	O
Macintosh	B-Device
Plus	I-Device
in	O
January	O
1986	O
,	O
would	O
accommodate	O
far	O
faster	O
and	O
more	O
efficient	O
hard	B-Device
drives	I-Device
,	O
rendering	O
the	O
Hard	B-Device
Disk	I-Device
20	I-Device
virtually	O
obsolete	O
.	O
</s>
<s>
The	O
Hard	B-Device
Disk	I-Device
20	I-Device
(	O
or	O
HD20	O
,	O
as	O
it	O
was	O
known	O
colloquially	O
)	O
contains	O
a	O
20	O
MB	O
3.5	O
"	O
Rodime	O
hard	B-Device
disk	I-Device
which	O
provides	O
over	O
50	O
times	O
the	O
data	O
storage	O
of	O
the	O
stock	O
400	O
kB	O
disk	O
drive	O
.	O
</s>
<s>
At	O
the	O
time	O
when	O
the	O
average	O
file	O
size	O
was	O
around	O
10-20	O
kB	O
and	O
due	O
to	O
the	O
vast	O
number	O
of	O
those	O
files	O
the	O
HD20	O
can	O
contain	O
,	O
Apple	O
's	O
original	O
Macintosh	B-Application
File	I-Application
System	I-Application
,	O
which	O
does	O
not	O
allow	O
for	O
directories	O
,	O
made	O
organizing	O
those	O
files	O
unwieldy	O
.	O
</s>
<s>
Therefore	O
,	O
Apple	O
introduced	O
it	O
with	O
a	O
new	O
System	O
and	O
Finder	O
update	O
which	O
include	O
the	O
Hierarchical	B-Application
File	I-Application
System	I-Application
allowing	O
the	O
user	O
to	O
better	O
organize	O
files	O
on	O
such	O
a	O
large	O
volume	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
only	O
the	O
Macintosh	B-Device
512K	I-Device
can	O
access	O
it	O
;	O
the	O
original	B-Device
Macintosh	I-Device
128K	I-Device
does	O
not	O
have	O
enough	O
RAM	B-Architecture
to	O
load	O
the	O
new	O
file	O
system	O
.	O
</s>
<s>
In	O
fact	O
,	O
even	O
for	O
the	O
Macintosh	B-Device
512K	I-Device
to	O
use	O
the	O
drive	O
,	O
it	O
requires	O
an	O
additional	O
file	O
in	O
the	O
System	O
Folder	O
on	O
a	O
special	O
startup	O
disk	O
which	O
adds	O
additional	O
code	O
into	O
memory	O
during	O
startup	O
.	O
</s>
<s>
A	O
startup	O
routine	O
also	O
allows	O
the	O
Mac	O
to	O
check	O
for	O
the	O
presence	O
of	O
a	O
System	O
file	O
on	O
the	O
Hard	B-Device
Disk	I-Device
,	O
switch	O
over	O
to	O
it	O
and	O
eject	O
the	O
startup	O
disk	O
.	O
</s>
<s>
With	O
the	O
release	O
of	O
the	O
Macintosh	B-Device
Plus	I-Device
and	O
the	O
Macintosh	B-Device
512Ke	I-Device
,	O
both	O
containing	O
the	O
upgraded	O
128	O
kB	O
ROM	B-Device
which	O
contains	O
the	O
additional	O
code	O
,	O
the	O
HD20	O
can	O
finally	O
be	O
used	O
alone	O
as	O
a	O
startup	O
disk	O
.	O
</s>
<s>
While	O
other	O
hard	B-Device
drives	I-Device
were	O
available	O
on	O
the	O
market	O
,	O
Apple	O
's	O
HD20	O
was	O
generally	O
preferred	O
because	O
it	O
uses	O
the	O
high-speed	O
floppy	O
disk	O
port	O
,	O
whereas	O
third-party	O
drives	O
use	O
the	O
lower-speed	O
Serial	B-Protocol
Port	I-Protocol
.	O
</s>
<s>
While	O
the	O
floppy	O
port	O
was	O
not	O
initially	O
supposed	O
to	O
be	O
used	O
this	O
way	O
,	O
making	O
creative	O
use	O
of	O
the	O
port	O
allows	O
the	O
HD20	O
to	O
achieve	O
much	O
higher	O
transfer	O
rates	O
than	O
products	O
limited	O
to	O
the	O
serial	B-Protocol
ports	I-Protocol
.	O
</s>
<s>
With	O
few	O
exceptions	O
,	O
this	O
along	O
with	O
complete	O
compatibility	O
with	O
the	O
new	O
Hierarchical	B-Application
File	I-Application
System	I-Application
,	O
gave	O
Apple	O
an	O
instant	O
edge	O
over	O
the	O
competition	O
.	O
</s>
<s>
In	O
addition	O
,	O
the	O
HD20	O
has	O
a	O
convenient	O
"	O
zero-footprint	O
"	O
design	O
which	O
fit	O
precisely	O
underneath	O
the	O
Macintosh	B-Device
,	O
merely	O
elevating	O
it	O
3	O
inches	O
,	O
but	O
otherwise	O
taking	O
up	O
no	O
more	O
desk-space	O
.	O
</s>
<s>
In	O
1985	O
,	O
the	O
HD20	O
was	O
an	O
important	O
step	O
to	O
solidifying	O
the	O
Macintosh	B-Device
as	O
a	O
true	O
business	O
computer	O
and	O
it	O
was	O
eagerly	O
anticipated	O
following	O
its	O
April	O
announcement	O
.	O
</s>
<s>
Until	O
Apple	O
's	O
introduction	O
a	O
year	O
later	O
of	O
the	O
Hard	B-Device
Disk	I-Device
20SC	I-Device
,	O
the	O
first	O
SCSI	B-Architecture
drive	O
they	O
manufactured	O
,	O
the	O
HD20	O
was	O
the	O
only	O
Apple-manufactured	O
hard	B-Device
drive	I-Device
available	O
for	O
any	O
Macintosh	B-Device
except	O
the	O
Macintosh	B-Device
XL	I-Device
.	O
</s>
<s>
However	O
,	O
the	O
HD20	O
's	O
unique	O
design	O
and	O
position	O
in	O
the	O
marketplace	O
was	O
quickly	O
outmoded	O
by	O
the	O
advancement	O
of	O
the	O
significantly	O
faster	O
SCSI	B-Architecture
standard	O
which	O
debuted	O
with	O
the	O
Macintosh	B-Device
Plus	I-Device
in	O
January	O
1986	O
.	O
</s>
<s>
Some	O
third	O
party	O
companies	O
offered	O
a	O
SCSI	B-Architecture
conversion	O
kit	O
which	O
replaced	O
the	O
controller	O
board	O
thus	O
preserving	O
the	O
user	O
's	O
investment	O
in	O
the	O
expensive	O
but	O
proprietary	O
Rodime	O
drive	O
.	O
</s>
<s>
Apple	O
officially	O
dropped	O
support	O
for	O
the	O
HD20	O
with	O
System	B-Protocol
6	I-Protocol
as	O
well	O
as	O
omitting	O
the	O
necessary	O
ROM	B-Device
code	O
beginning	O
with	O
the	O
Macintosh	B-Device
II	I-Device
.	O
</s>
<s>
Sales	O
of	O
the	O
HD20	O
continued	O
to	O
support	O
the	O
Macintosh	B-Device
512Ke	I-Device
which	O
has	O
no	O
other	O
hard	B-Device
drive	I-Device
options	O
,	O
until	O
it	O
was	O
discontinued	O
in	O
late	O
1987	O
.	O
</s>
<s>
Only	O
Macs	O
with	O
legacy	O
technology	O
and	O
floppy	O
disk	O
ports	O
,	O
which	O
were	O
eliminated	O
entirely	O
from	O
Macintosh	B-Device
computers	O
in	O
1991	O
,	O
are	O
able	O
to	O
continue	O
to	O
use	O
the	O
older	O
slower	O
technology	O
.	O
</s>
<s>
Manufactured	O
in	O
significant	O
numbers	O
for	O
almost	O
two	O
years	O
,	O
the	O
HD20	O
remains	O
as	O
one	O
of	O
the	O
few	O
surviving	O
hard	B-Device
drives	I-Device
usable	O
by	O
a	O
stock	O
Macintosh	B-Device
512K	I-Device
or	O
512Ke	O
.	O
</s>
