You might want to boot from a USB device, like an external hard drive or a flash drive, for many different reasons.
When you boot from a USB device, what you're actually doing is running your computer with the operating system that's installed on the USB device. When you start your computer normally, you're running with the operating system installed on your hard drive - Windows, Linux, etc.
How To Boot From a USB Device
Change the BIOS boot order so the USB device option is listed first. The BIOS is rarely setup this way by default.
If the USB boot option is not first in the boot order, your PC will start "normally" (i.e. boot from your hard drive) without even looking at any boot information that might be on your USB device.
Tip: The BIOS on most computers list the USB boot option as USB or Removable Devices but some confusingly list it as a Hard Drive option, so be sure to dig around if you're having trouble finding the right one to choose.
Note: After setting your USB device as the first boot device, your computer will check it for boot information each time your PC starts. Leaving your computer configured this way shouldn't cause problems unless you plan on leaving the bootable USB device attached all the time.
Attach the USB device to your computer via any available USB port.
Note: Creating a bootable flash drive or configuring an external hard drive as bootable is a task in itself. Chances are you made it to my instructions here because you know whatever USB device you have should be bootable after properly configuring BIOS.
Note: Most of the time when trying to boot to a USB device there is no key-press prompt. The USB boot process usually starts immediately.
- Restart your computer.
- Watch for a Press any key to boot from external device... message.
- On some bootable devices, you may be prompted with a message to press a key before the computer will boot to the flash drive or other USB device.
- If you do nothing, your computer will check for boot information on the next boot device in the list in BIOS (see Step 1) which will probably be your hard drive.
- Your computer should now boot from the flash drive or USB based external hard drive.