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.
For this how-to, I chose Mountain Lion. You will need to have a copy of the Mountain Lion installation app, purchased from the App Store. After you click the "Mountain Lion (10.8)" button, the app will search your home directory for the install app. You can use the copy it finds or you can choose a different copy. I chose the copy that it found and clicked the "Use this copy" button.