Some BIOSes will, oddly enough, boot USB optical drives but not USB mass storage. No idea why, it may have to do with the fact that way too many USB flash drives have piss-poor SCSI command support (they speak SCSI Block Commands, USB is just used as transport).

Here's a hilarious list of common problems in USB sticks: http://www2.one-eyed-alien.net/~mdharm/linux-usb/target_offe...