What Files Are Needed to Boot From a USB?by Blair Williams
Not every PC is capable of booting from a flash drive, nor is every flash drive capable of booting software on a PC. But booting from a handy little flash drive is more than just a convenience. If your machine won't start at all, it could be a computer lifesaver. With the newest operating systems, it's easier than ever to bypass a hard-drive boot and load from a USB device.
Before you can boot from a USB flash drive on a PC, you need to check your computer's BIOS. For a few seconds after you turn on your computer, you will see a message on your screen that will tell you what key to press to enter the BIOS. Not every computer has the same message, nor is the key to press always the same between machines. Look for words like "BIOS setup" or "setup" or "system configuration" and press the corresponding key to gain access.
Once you've accessed your BIOS, find the boot-sequence menu. Newer computers may display an option that says something like "USB Flash Drive" or "USB Storage." You want to choose that option, which will make your computer load from the USB stick instead of your hard drive. If you don't see the USB flash-drive option in the menu, exit from the BIOS, turn off your computer and insert your USB stick. Then reboot, enter the BIOS and check the boot-sequence menu again. If you still don't see an option for a USB drive, your computer isn't capable of booting this way.
Configuring your USB drive to load Windows 7 is less convoluted than setting up your USB flash drive to boot XP. Microsoft has developed software you can download to automatically configure your USB drive to boot 7, called the Windows 7 USB/DVD Download Tool. For XP, you'll need to format the flash drive, then make it bootable. After those two steps you'll need to configure Windows Setup to load from a flash drive; then you can begin your install. Detailed instructions for both operating systems can be found in the Resources link below.
Mac OS X
All versions of OS X can be installed via a USB flash drive. As of 2010, the installation files for the latest Snow Leopard version is a little more than 6 GB, so you'll need a USB with a storage capacity of 8 GB. Use the Disk Utility tool to copy the Snow Leopard files from your installation DVD to your USB. Then remove your DVD, reboot and hold down option once your computer restarts. You'll be given the option to boot from your USB drive, which you'll choose, and the install begins.
- USB image by Ewe Degiampietro from Fotolia.com