In order to set an external hard drive as the main hard drive of your computer, you need to make a few changes to the computer's BIOS. These changes are helpful if your internal hard drive has failed or has become corrupted. You can also use this method to circumvent the need for a boot manager, and simply use various external hard drives, loaded with different operating systems, to boot your computer.
Prepare the USB drive. Install your operating system of choice to the USB drive. Also install any necessary drivers, their updates and operating system updates.
Prepare your computer. Access your computer's BIOS and navigate to the Boot Order menu. This lists all available boot devices. Using the commands listed on the BIOS screen, select the option that resembles "USB device" from the list and move that to the top position.
Shut down your computer.
Attach your external USB hard drive. Plug this drive in to any of the available USB ports. If you are permanently attaching the drive to a desktop, use a rear USB port to protect the cable.
Test the USB hard drive. Boot your computer to ensure the USB hard drive is successfully recognized. Also check the drive letter assignments for the built in hard drive and the CD-ROM (if present).
The method for accessing the BIOS is different for each computer. The boot screen usually provides the proper key or key sequence. Typically, you press F8, F12 or DEL.
If your BIOS does not offer the option of booting from USB, check the manufacturer's website for a firmware upgrade that may allow the option.
Booting from a USB hard drive requires a basic understanding of your computer's BIOS. Do not attempt to do this if you are unfamiliar with the BIOS and its settings.
Improper edits to the BIOS may result in unexpected behavior.