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.
or open the boot device menu (most manufacturers use F12 during the POST screen, others may use ESC or a combination) and choose the device you want to from.
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