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.
During disk creation you'll see the system mount a number of different disk images. If you look for them in Finder, you'll be able to see that it's grabbing important system apps and applets and holding them in a temporary location so the contents can be copied to your thumb drive.