If you’ve followed our previous instructions on installing OS X Mountain Lion on your Mac, you have a backup of the Mountain Lion installer somewhere on a disk lying around. You can take this disk to any OS X computer running Lion and run the installer over there to upgrade it to Mountain Lion. If, on the other hand, you have an older Mac with a version of OS X that cannot be upgraded directly to Mountain Lion, or you want to do a clean re-install of the OS, this is the guide you should follow to create a bootable Mountain Lion USB Thumb Drive.
Search the drawers for an 8GB Thumb Drive. The Mountain Lion installer is about 5GB so anything more than that should do. Insert the key into your Mac.
Now, find the Mountain Lion installer that you copied away safely earlier. If you don’t have it, download it again from the Mac App Store’s Purchases section.
When the download completes, the installer will launch. Quit it.
Use the Finder to browse over to the Applications folder if you just downloaded the installer or the folder where you copied the previously downloaded installer.
Right click the Installer icon and select the Show Package Contents option.
Browse to the Contents/Shared Support folder.
There will be two files in this folder. We are interested in the DMG file named InstallESD.dmg.
Right-click this file and select the Open With->Disk Utility option.
This will open Disk Utility and you should be able to see your USB key as well as the InstallESD DMG file in the left side column.
Select the USB key and click on the Partition tab.
From the Partition Layout tab, select 1 Partition and then click the Options button below the partition layout graphic.
Make sure that the GUID Partition Table radio button is selected. Click Ok and hit Apply. If Disk Utility warns you about your disk being erased, click Ok and let it proceed.
Now, select and right click the InstallESD DMG file in the left column and choose the Open Disk Image option.
Select the newly mounted Mac OS X Install ESD volume and click on the Restore tab on the right side.
The Source field in the Restore tab should now be set to Mac OS X Install ESD. In the Destination field, drag and drop the partition that we earlier created on the USB Key and then press the Restore button.
This will copy the contents of the Installer image to the USB key.
You can now use this USB key as a bootable installer on any Mac.