How To Create An OS X Mountain Lion Install USB Drive
Here's how to use a free utility, Lion Disk Maker, to create a boot drive for clean installations or emergency use on your Lion- or Mountain Lion-compatible Mac. Create a bootable USB thumb drive like we did, or a bootable DVD.
The app assumes that you have already inserted your thumb drive into your Mac and have appropriately initialized, formatted, partitioned, and named the drive. Choose the drive from the drop-down list and click the "Choose this disk" button.