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.
It's strongly recommended that you use an 8GB thumb drive. The contents of the disk image you're going to be burning are just over 4GB, so the 4GB version that might have worked for Lion won't work for Mountain Lion. Click the "An 8 GB USB thumb drive" button.