<s>
A	O
forensic	B-Device
disk	I-Device
controller	I-Device
or	O
hardware	B-Device
write-block	I-Device
device	I-Device
is	O
a	O
specialized	O
type	O
of	O
computer	O
hard	B-General_Concept
disk	I-General_Concept
controller	I-General_Concept
made	O
for	O
the	O
purpose	O
of	O
gaining	O
read-only	O
access	O
to	O
computer	O
hard	B-Device
drives	I-Device
without	O
the	O
risk	O
of	O
damaging	O
the	O
drive	O
's	O
contents	O
.	O
</s>
<s>
The	O
device	O
is	O
named	O
forensic	O
because	O
its	O
most	O
common	O
application	O
is	O
for	O
use	O
in	O
investigations	O
where	O
a	O
computer	O
hard	B-Device
drive	I-Device
may	O
contain	O
evidence	O
.	O
</s>
<s>
Such	O
a	O
controller	O
historically	O
has	O
been	O
made	O
in	O
the	O
form	O
of	O
a	O
dongle	B-Device
that	O
fits	O
between	O
a	O
computer	O
and	O
an	O
IDE	B-Protocol
or	O
SCSI	B-Architecture
hard	B-Device
drive	I-Device
,	O
but	O
with	O
the	O
advent	O
of	O
USB	B-Protocol
and	O
SATA	O
,	O
forensic	B-Device
disk	I-Device
controllers	I-Device
supporting	O
these	O
newer	O
technologies	O
have	O
become	O
widespread	O
.	O
</s>
<s>
Steve	O
Bress	O
and	O
Mark	O
Menz	O
invented	O
hard	B-Device
drive	I-Device
write	O
blocking	O
(	O
US	O
Patent	O
6,813,682	O
)	O
.	O
</s>
<s>
A	O
device	O
which	O
is	O
installed	O
between	O
a	O
storage	O
media	O
under	O
investigation	O
and	O
an	O
investigator	O
's	O
computer	O
is	O
called	O
a	O
"	O
bridge	B-Device
kit	I-Device
"	O
.	O
</s>
<s>
The	O
bridge	B-Device
kit	I-Device
has	O
one	O
connector	O
for	O
the	O
storage	O
media	O
and	O
another	O
connector	O
the	O
investigator	O
's	O
computer	O
.	O
</s>
<s>
Forensic	B-Device
disk	I-Device
controllers	I-Device
intercept	O
write	O
commands	O
from	O
the	O
host	O
operating	B-General_Concept
system	I-General_Concept
,	O
preventing	O
them	O
from	O
reaching	O
the	O
drive	O
.	O
</s>
<s>
Whenever	O
the	O
host	O
bus	B-General_Concept
architecture	O
supports	O
it	O
the	O
controller	O
reports	O
that	O
the	O
drive	O
is	O
read-only	O
.	O
</s>
<s>
The	O
disk	B-General_Concept
controller	I-General_Concept
can	O
either	O
deny	O
all	O
writes	O
to	O
the	O
disk	O
and	O
report	O
them	O
as	O
failures	O
,	O
or	O
use	O
on-board	O
memory	O
to	O
cache	O
the	O
writes	O
for	O
the	O
duration	O
of	O
the	O
session	O
.	O
</s>
<s>
A	O
disk	B-General_Concept
controller	I-General_Concept
that	O
caches	O
writes	O
in	O
memory	O
presents	O
the	O
appearance	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
that	O
the	O
drive	O
is	O
writable	O
,	O
and	O
uses	O
the	O
memory	O
to	O
ensure	O
that	O
the	O
operating	B-General_Concept
system	I-General_Concept
sees	O
changes	O
to	O
the	O
individual	O
disk	O
sectors	O
it	O
attempted	O
to	O
overwrite	O
.	O
</s>
<s>
It	O
does	O
this	O
by	O
retrieving	O
sectors	O
from	O
the	O
disk	O
if	O
the	O
operating	B-General_Concept
system	I-General_Concept
has	O
n't	O
attempted	O
to	O
change	O
them	O
,	O
and	O
retrieving	O
the	O
changed	O
version	O
from	O
memory	O
for	O
sectors	O
that	O
have	O
been	O
changed	O
.	O
</s>
<s>
Forensic	B-Device
disk	I-Device
controllers	I-Device
are	O
most	O
commonly	O
associated	O
with	O
the	O
process	O
of	O
creating	O
a	O
disk	O
image	O
,	O
or	O
acquisition	O
,	O
during	O
forensic	O
analysis	O
.	O
</s>
<s>
Using	O
hardware	O
to	O
protect	O
the	O
hard	B-Device
drive	I-Device
from	O
writes	O
is	O
very	O
important	O
for	O
several	O
reasons	O
.	O
</s>
<s>
First	O
,	O
many	O
operating	B-General_Concept
systems	I-General_Concept
,	O
including	O
Windows	B-Application
,	O
may	O
write	O
to	O
any	O
hard	B-Device
disk	I-Device
that	O
is	O
connected	O
to	O
the	O
system	O
.	O
</s>
<s>
At	O
the	O
very	O
least	O
,	O
Windows	B-Application
will	O
update	O
the	O
access	B-General_Concept
time	I-General_Concept
for	O
any	O
file	O
accessed	O
,	O
and	O
may	O
write	O
things	O
to	O
the	O
disk	O
unexpectedly	O
-	O
such	O
as	O
creating	O
hidden	O
folders	O
for	O
the	O
recycle	B-Application
bin	I-Application
or	O
saved	O
hardware	O
configuration	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
NTFS	B-Application
file	I-Application
system	I-Application
may	O
attempt	O
to	O
commit	O
or	O
rollback	O
unfinished	O
transactions	O
,	O
and/or	O
change	O
flags	O
on	O
the	O
volume	O
to	O
mark	O
it	O
as	O
"	O
in	O
use	O
"	O
.	O
</s>
<s>
At	O
the	O
worst	O
,	O
undesired	O
files	O
may	O
allocate	O
and	O
overwrite	O
deleted	O
space	O
on	O
the	O
hard	B-Device
disk	I-Device
which	O
may	O
potentially	O
destroy	O
evidence	O
in	O
the	O
form	O
of	O
previously	O
deleted	O
files	O
.	O
</s>
