check this thread......
viewtopic.php?f=50&t=9822If your installer can "see" your usb drive you can install to it,but you need to check your boot devices in your bios to see if it list your usb drive.You can check it during start up at the first screen .On my computer it says "HP" and has some function keys at the bottom, and press "esc" for boot.If I press it I see my Hdd then my usb drive and then my dvd-rom.Your computer may have something different like f8 or f12 or something else.I would run the liveDVD and see how sabayon labels your usb drive so you will know it in the installer.
If your bios doesn't have your drive you can download supergrub , make a cd of that and it will boot anything....