<s>
A	O
USB	B-Operating_System
image	I-Operating_System
—	O
is	O
bootable	O
image	B-General_Concept
of	O
Operating	B-General_Concept
system	I-General_Concept
(	O
OS	B-General_Concept
)	O
or	O
other	O
software	O
where	O
the	O
boot	B-Application
loader	I-Application
is	O
located	O
on	O
a	O
USB	B-Protocol
flash	O
drive	O
,	O
or	O
another	O
USB	B-Protocol
device	I-Protocol
(	O
with	O
memory	O
storage	O
)	O
instead	O
of	O
conventional	O
CD	B-Device
or	O
DVD	O
discs	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
loads	O
from	O
the	O
USB	B-Protocol
device	I-Protocol
either	O
to	O
load	O
it	O
much	O
like	O
a	O
Live	O
CD	B-Device
that	O
runs	O
OS	B-General_Concept
or	O
any	O
other	O
software	O
from	O
the	O
storage	O
or	O
installs	O
OS	B-General_Concept
itself	O
.	O
</s>
<s>
USB	B-Operating_System
image	I-Operating_System
runs	O
off	O
of	O
the	O
USB	B-Protocol
device	I-Protocol
the	O
whole	O
time	O
.	O
</s>
<s>
A	O
USB	B-Operating_System
image	I-Operating_System
is	O
easier	O
to	O
carry	O
,	O
can	O
be	O
stored	O
more	O
safely	O
than	O
a	O
conventional	O
CD	B-Device
or	O
DVD	O
.	O
</s>
<s>
Drawbacks	O
are	O
that	O
some	O
older	O
devices	O
may	O
not	O
support	O
USB	B-Protocol
booting	O
and	O
that	O
the	O
USB	B-Protocol
storage	I-Protocol
devices	I-Protocol
lifespan	O
might	O
be	O
shortened	O
.	O
</s>
<s>
Ubuntu	O
has	O
included	O
a	O
utility	O
for	O
installing	O
an	O
operating	B-General_Concept
system	I-General_Concept
image	B-General_Concept
file	O
to	O
a	O
USB	B-Protocol
flash	O
drive	O
since	O
version	O
9.10	O
.	O
</s>
<s>
Windows	O
support	O
also	O
has	O
added	O
a	O
step	O
by	O
step	O
on	O
how	O
to	O
set	O
up	O
a	O
USB	B-Protocol
device	I-Protocol
as	O
a	O
bootable	O
drive	O
.	O
</s>
<s>
Both	O
graphical	O
applications	O
and	O
command	O
line	O
utilities	O
are	O
available	O
for	O
authoring	O
bootable	O
operating	B-General_Concept
system	I-General_Concept
images	O
.	O
</s>
<s>
dd	B-Application
is	O
a	O
utility	O
commonly	O
found	O
in	O
Unix	B-Application
operating	I-Application
systems	I-Application
that	O
allow	O
creation	O
of	O
bootable	O
images	O
.	O
</s>
<s>
In	O
contrast	O
to	O
live	O
CDs	B-Device
,	O
a	O
USB	B-Operating_System
image	I-Operating_System
is	O
easier	O
to	O
transport	O
and	O
to	O
store	O
(	O
e.g.	O
</s>
<s>
The	O
absence	O
of	O
moving	O
parts	O
in	O
USB	B-Protocol
flash	O
devices	O
allows	O
true	O
random	B-General_Concept
access	I-General_Concept
avoiding	O
the	O
rotational	O
latency	O
and	O
seek	O
time	O
,	O
meaning	O
small	O
programs	O
will	O
start	O
faster	O
from	O
a	O
USB	B-Protocol
flash	O
drive	O
than	O
from	O
a	O
local	O
hard	O
disk	O
or	O
live	O
CD	B-Device
.	O
</s>
<s>
However	O
,	O
as	O
USB	B-Protocol
devices	I-Protocol
typically	O
achieve	O
lower	O
data	O
transfer	O
rates	O
than	O
internal	O
hard	O
drives	O
,	O
booting	O
from	O
older	O
computers	O
that	O
lack	O
USB	B-Protocol
2.0	I-Protocol
or	O
newer	O
can	O
be	O
very	O
slow	O
.	O
</s>
<s>
Some	O
older	O
systems	O
have	O
limited	O
support	O
for	O
USB	B-Protocol
,	O
since	O
their	O
BIOSes	O
were	O
not	O
designed	O
with	O
such	O
purpose	O
at	O
the	O
time	O
.	O
</s>
<s>
Other	O
devices	O
may	O
not	O
be	O
booted	O
from	O
USB	B-Protocol
,	O
if	O
in	O
BIOS	B-Operating_System
it	O
is	O
set	O
to	O
'	O
Legacy	O
mode'Legacy	O
mode	O
.	O
</s>
<s>
Due	O
to	O
the	O
additional	O
write	O
cycles	O
that	O
occur	O
on	O
a	O
full	O
installation	O
,	O
the	O
life	O
span	O
of	O
the	O
used	O
USB	B-Protocol
may	O
be	O
shortened	O
.	O
</s>
<s>
To	O
mitigate	O
this	O
,	O
a	O
USB	B-Protocol
hard	O
drive	O
can	O
be	O
used	O
,	O
as	O
they	O
give	O
better	O
performance	O
than	O
the	O
USB	B-Protocol
stick	O
,	O
regardless	O
of	O
the	O
connector	O
.	O
</s>
