Sorry if this detail's been covered, but; not all OSs will be bootable
or can operate via a USB (or Firewire?) interface.
Vista may, but as XP initializes the USB during the boot process, it
will saw its legs off. The first code base that didn't do that was
Windows Server 2003, either "gold" or SP1. This has significance for
folks trying to build bootable Bart USB sticks.
OTOH...
....Format /S installs DOS as the booting OS, and DOS can't "see" USB
or Firewire either (though it may do if BIOS already "sees" it as a
bootable interface, dunno if that's enough for success).
The NT family of OSs (NT, 2000, XP, 2003, Vista) don't contain DOS as
an alternate maintenance OS or boot, as Win9x did - they use WinPE as
the maintenance OS (available to shlebs like us only in the Vista
age). Up until XP, the ability to format diskettes as DOS boot disks
was preserved, but has prolly gone away in Vista.
There are good reasons to "retire" DOS, chief among these being safety
issues > 137G, inability to read NTFS and USB, etc. WinPE 2.0 is not
only available to all (though not exactly on a plate, it is in fact
the mOS that a Vista DVD will boot into when "Repair, Command prompt"
is selected), it is not actively promoted as replacement for DOS for
off-HD system maintenance contexts. At last!