The usual way to install an operating system is to insert a DVD or CD into your computer and let the process take its course. This is fine for the initial install, but what happens if you need to re-install your operating system and find that you've lost the disc, or that the disc is scratched? Fortunately, you can avoid paying twice for your OS by using a USB flash drive as a backup. Doing this requires some comfort level with computers, but it isn't overly complicated.
Instructions
Windows
1. Open the Command Prompt from the "Start" menu. Type "diskpart" and press "Enter." Type "list disk" and press "Enter." Check the list of attached media to find the number of your flash drive. The storage capacity will be the best clue.
2. Type "select disk x", replacing "x" with the number of your flash drive, then type "Clean" and press "Enter." Type the following commands, pressing "Enter" after each one: "Create partition primary," "Select partition 1," "active," "Format fs=fat32," "Assign," and "Exit."
3. Type the following command: "xcopy d:*.* /s/e/f g:" Replace "d" with the drive letter of your DVD drive and "g" with the drive letter for your USB flash drive. Press "Enter" and wait for the files to copy. When this finishes, your USB flash drive is now bootable.
4. Restart your computer and, as soon as it turns back on, open the BIOS setup. (This step varies by computer manufacturer. You'll need to press a key while the initial startup screen is visible. Common keys include "F2" and "Delete." The key should be listed on the initial screen.)
5. Find the "Boot" section of the BIOS interface. You'll see a list of all possible bootable devices attached to the computer, listed in descending order of priority. Thus, your hard drive will probably be listed first, since your computer usually boots from there. Other possible devices may include the DVD drive, the Local Area Network, and your USB flash drive.
6. Move the USB flash drive to the top of the boot list. Now your computer will always check the USB flash drive before booting from the hard drive. So if you need to re-install your OS and the DVD or CD is missing or damaged, you can plug in your USB flash drive and follow the instructions that appear.
Mac OS X
7. Insert your OS X DVD and USB flash drive, then select "Utility" from the menu bar. Select "Disk Utility."
8. Click on your USB flash drive, then click "Partition" at the top of the window. Select "1 Partition" from the "Volume Select" menu.
9. Click "Options", then select "GUID Partition Table" (if using an Intel-based Mac) and click "OK."
10. Return to the "Disk Utility" main screen and select "Erase" from the options at the top of the window. Select "Mac OS X Extended" from the "Volume Format" menu and give the volume a name you can remember. Click the "Erase" button located below.
11. Select "Restore" from the options at the top of the window. Drag your OS X disc icon to the "Source" field. Drag your USB flash drive to the "Destination" field and click "Restore." Wait for this process to finish.
12. Remove the OS X DVD and restart. When the computer first powers up, hold down the "Option" key. Select your USB drive from the list that appears. The installation process will start automatically. Follow the on-screen instructions.
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