Re: i386 or amd64?
So it sounds like either that bios is incapable of booting from usb or it is possibly disabled somewhere else in the bios setup. I've had one motherboard that you actually had to move a jumper on the board to enable usb boot.
Try to Google your motherboard model number and obtain the manual to it, find out if the motherboard supports usb boot and/or how to enable that ability. Maybe even check for bios updates, there might be one that enables that feature.
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