Windows can be forced to boot via USB drive on a reboot by configuring the computer to prioritize USB devices over hard drives. Windows users may boot from a USB drive to install the operating system on a computer without an optical drive. Traditionally, the boot selection process is handled through the computer's BIOS configuration or by launching the boot selector menu when first turning on the computer. However, computers made after the release of Windows 8 start loading Windows too quickly for the user to activate the menu with a keyboard shortcut.

Windows 8 Recovery Environment

Windows 8 lets users select an alternative boot device like a USB drive in the Recovery Environment boot menu. Usually, the Recovery Environment is used when something has gone wrong with the computer; however, there are two ways to access the Recovery Environment deliberately. Connect the USB device to the computer before using either method. To use the first method, open the Windows shutdown menu and select "Restart" while holding down the "Shift" key. The second method is to use the "Advanced Startup" option by searching for "Advanced Startup" in the Charms search menu, selecting "Advanced Startup" and clicking "Restart Now."

Windows 8 Boot Device Selection

When deliberately triggering the Recovery Environment, the computer will start to boot like normal and display the BIOS splash screen: however, the computer will load into the Recovery Environment instead of Windows 8. After the Recovery Environment is finished loading, select the "Use a device" option, click "Boot Menu" and select the USB drive from the list. Older computers may not support the "use a device" option and require using BIOS to reconfigure boot priority.

Legacy BIOS Boot Priority

Older Windows 8 running computers have a delay during the BIOS splash screen during which you can press a keyboard button to access the system configuration. If USB devices are prioritized above hard drives, the computer will automatically boot via USB when a USB device is connected. The screen will usually display which key you'll need to press on the splash screen to enter BIOS. The key is usually "F1," F2," F10," "Del" or "Esc." Once in BIOS, use the keyboard arrow keys to navigate to the "Boot" menu. Following the onscreen instructions, change the first priority boot device to "USB" and select "Save and Exit." When the computer restarts it will boot from the connected USB device.

USB Drive Must be Bootable

Simply copying a program to a USB drive and selecting it from a boot option will not work. A USB device needs to have a master boot record in the boot sector for a computer to be able to boot from it. The boot sector includes the necessary instructions for a computer to boot to an operating system or program on a storage device. Formatting the drive for boot requires using the command line prompt as an administrator.