How can I boot from an external hard drive?
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You need to enter the BIOS of your computer and select USB as first boot device.
Very often, the BIOS is entered by pressing a key at the very beginning of the boot sequence, right after having powered the computer on:
Once you're in the BIOS, you can configure the boot order: for instance try to boot from floppy first, then cdrom/dvdrom, then hard-drive... The boot order is stored in the BIOS remains the same every time the computer starts.
As suggested in other answers and comments, some BIOS allow you to press a key (F12 for instance) that will bring you a one shot boot menu: the purpose of this menu is to let you exceptionally boot from another device.
If you exceptionally want to boot from an USB hard-drive, you're likely to want to bring the boot menu up. If you want to always boot from an USB hard-drive that remains plugged to your computer you're likely to change the boot order in your BIOS.