I boot from an external 5.25" USB enclosure with a DVD drive
in it. I boot from my collection of Knoppix Linux LiveCDs, via
that external drive, or by using the internal drive inside my
computer. Either will work.
I have also booted from a USB flash, where a 1440KB image of a
DOS floppy was copied to the flash drive with "dd". So somehow, my
BIOS was able to boot from that (I never expected it to work).
The USB module in the BIOS, may have basic booting abilities,
as well as certain emulation modes, to convert a device into
something usable. But chances are, there is no documentation
about this.
There was a short interval of time (maybe a year), where
the BIOS had a separate page with details about what USB devices
were connected, their emulation mode if any and so on. The BIOS
writers have since made that invisible, and on newer computers
or motherboards, it no longer gets its own screen.
Note that, if you take your WinXP hard drive from inside the computer,
and place it into an external USB enclosure, it will not boot properly.
There is a problem with unmodified WinXP, in that the USB chain
gets disconnected during the boot sequence, and that prevents
the boot from completing (I think that is what gets broken).
Some clever people have figured out how to fix it, but the recipe
is not particularly easy. Some other OSes are more friendly with
respect to booting and operating from USB.
So one of the elements to being bootable, is whether the OS
can maintain the connection over USB, while booting is completing.
The hardest part about USB booting, is figuring out how to
make a bootable device. For example, it took me a while to
get something to work with my USB flash sticks, and my
first few attempts were not successful. Even a Knoppix option,
to make a bootable Knoppix environment on a flash stick,
didn't work (it would not boot for me). So there are a
number of recipes for making bootable devices, which may
or may not work. There are too many variables,
to predict with any certainty what will work.
I could not find this document on ami.com , so had to find it on another site.
This shows what options used to exist. See section 2.5.8.1
on PDF page 8.
http://www.securitytechnet.com/resource/hot-topic/homenet/AMIBIOS8_US...
OK. Found it on archive.org as well.
http://web.archive.org/web/20030308133556/http://www.ami.com/support/...
Paul