Install OS X from USB drive

Like other versions of OS X, Snow Leopard can be installed from a USB drive. This is especially beneficial to Macbook Air users looking to install the OS without a DVD drive, or anyone else that might want to do this.

To do this, you’ll need to prepare your USB drive with the OS X installation files. The steps listed below worked particularly for Snow Leopard but should also work for installing any version of OS X from USB.

The installation files sit at 6.5gb on the Snow Leopard DVD, so you’ll probably need a drive that has at least 8gb available.

After you’ve gotten the correct USB device, connect it to your Mac and prepare it with the Snow Leopard installation files by following these steps:

1. Open Disk Utility and select the Flash Drive.
2. Select the Erase tab on the right and then set Format to Mac OS Extended (Journaled). Name the partition you’ll be creating Snow Leopard or OS X Install so you can keep track of your installation device. After you’ve done this, click the Erase button.
3. After Disk Utility finishes erasing your old data on the flash drive and setting it as a new partition, it will be ready for the Snow Leopard install files. Select the Restore tab at the top and then drag the Snow Leopard DVD to the Source field. Select your USB device for the Destination file. 
4. Click Restore and wait for the restoration to finish (depending on the speed of your  USB drive it will take anywhere from about 20 to 30 minutes).
5. Now that the installation files are successfully on your USB device, your are ready to boot from it. Remove the Snow Leopard installation DVD from your drive. Next, restart your Mac and when it first starts to reboot, hold down the option (alt) key on your keyboard. After a few seconds at least two volumes should appear for you to select from for installation. One of those will be the USB drive we just prepared. Select it.
6. After the Snow Leopard installation software boots from your USB drive, follow the on screen instructions for installation. 
7. After the installation finishes, it will reboot your computer into your new upgraded version of OS X, version 10.6 Snow Leopard.

This method works for installing OS X Snow Leopard onto a Mac from a USB Drive. It should also work for installing Leopard or other older versions of OS X on Intel Based Macs.