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.
Follow these easy steps to boot from a flash drive, an external hard drive, or some other bootable USB device.
Time Required: Booting from a USB device usually takes less than 10 to 20 minutes.
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.
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.
See my How To Burn an ISO File to a USB Drive tutorial for general instructions on doing exactly that, which tends to be the the reason most people need to figure out how to boot from one.
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.
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.
Your computer should now boot from the flash drive or USB based external hard drive.
Note: What happens now depends on what the bootable USB device was intended for. If you're booting to Windows 8 or Windows 7 installation files on a flash drive, the operating system setup will begin. If you're booting to the DSL version of Linux, it will start. You get the idea.
If you tried the above steps but your computer did not boot from the USB device, check out some of the tips below:
Recheck the boot order in BIOS (Step 1). The number one reason a bootable flash drive or other USB device won't boot is because BIOS is not configured to check the USB port first.
Didn't find a "USB Device" boot order listing in BIOS? If your computer was manufactured around 2001 or before, it may not have this ability. If your computer is newer, check for some other ways that the USB option might be worded. In some BIOS versions, it's called "Removable Devices" or "External Devices".
Remove other USB devices. Other connected USB devices, like printers, external media card readers, etc., could be consuming too much power, or causing some other problem, which is preventing the computer from booting from flash drive or other device. Unplug all other USB devices and try again.
Switch to another USB port. The BIOS on some motherboards only check the first few USB ports. Switch to another USB port and restart your computer.
Copy the files to the USB device again. If you created the bootable flash drive or external hard drive yourself, which you probably did, repeat whatever steps you took again. You may have made a mistake during the process.
See How to Burn an ISO File to USB if you started with an ISO image. Getting an ISO file onto a USB drive, like a flash drive, isn't as easy as just expanding or copying the file there.