Having an external hard drive that is bootable has many different uses. The most popular uses are for computer troubleshooting and running alternative operating systems. With a bootable external drive, the computer can start up with an operating system that is independent from the main installed operating system, even if the two operating systems are the same type and version. To boot from an external hard drive, the computer must first recognise that the hard drive can be booted.
Power off the computer.
Plug in the external hard drive to an available port on the computer. If the external drive is also externally-powered, plug it into an electrical socket and turn the drive on.
Power on the computer.
Enter the BIOS set-up screen. This screen is accessible on most computers by pressing either the "F2" or "Del" key immediately after powering on the computer.
Navigate to the "Boot" section of the BIOS by using the arrow keys on the keyboard.
Set the external hard drive as the first drive in the boot order. Most BIOS set-up screens set the order by pressing the + and - keys. Refer to the computer owner's manual for different settings.
Exit the BIOS screen and save the settings. Most computer BIOS screens exit and save by pressing the "F10" key. Refer to the computer owner's manual for different settings.
Restart the computer. The computer is now set to see the external hard drive as the first boot device, instead of the computer's internal hard drive.
When the computer boots from the external hard drive, all configuration and operating system changes occur on the external hard drive and do not affect the operating system installed on the internal hard drive. An external hard drive must be pre-installed with a bootable operating system, such as Linux, before it can fully boot a system.