Mountain Lion USB Thumb Drive
Before you start
You will need a copy of the OS X Mountain Lion installer. You obtain this by downloading it from the Apple App Store on your Mac.
When the download is complete, please do not run the installer. Rather “quit” the installer. If you miss this step you will need to re-download the installer before you can begin
Start the process by navigating to your Applications folder, where you will find the installer. The file you’re looking for is simply called “Install OS X Mountain Lion”.
Right click (or Control + Click) the installer and select “Show Package Contents”, and navigate to the “Shared Support” folder, where you will find the installer “InstallESD.dmg”. I would suggest making a copy of this file, and making a backup for it, just in case something goes wrong. (I put a copy into another “backup” directory)
Open a new Finder windows (Command+N or File>New Finder Window) and Navigate to /Applications/Utilities/ and launch the “Disk Utility”
Attach your USB drive (should be 8GB or more). Once it’s attached and the machine has attached the drive, you will find it in your drive list. Select the drive, and choose the “Restore” tab. Drag InstallESD.dmg into the file-open window and click open – or select Image, and select it by navigating. Next, drag your partition to the “Destination:” location, and drop. Click “Restore” (you will likely need to authenticate as an Administrative user.)
Generally, this process will take about 15-30 minutes. Once your disk is complete, relaunch your installer, and you’re ready to go!