USB wont boot on some computers.

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https://bbs.archlinux.org – Some older bioses boot from USB in "floppy mode" (it shows the disk as a floppy if it is not partionned, otherwise the first partition is shown as a floppy, possibly with a limitation of size). If you know your bootloader, you can manage to make it bootable (I think this is much easier with syslinux than with grub, because it can detect at boot time from where it is booted; without having you to explicitly tell it). You can also try the plop boot manager (http://www.plop.at/en/bootmanager/index.html). It is able to boot USB devices in hard disk mode even without Bios support. o (HowTos)