Once your Mac’s hard drive is erased, quit Disk Utility and go back into the Mountain Lion installer.(Make sure you have a backup first, just in case.) From the erase tab, choose “Mac OS Extended (Journaled)” and wipe your drive. Inside the Mountain Lion installation tool, you can open Disk Utility and erase your Mac’s hard drive.Restart your Mac while holding down the option key.
For a clean install of Mountain Lion on any compatible Mac, follow these steps: Now that you have a bootable copy of Mountain Lion, this drive/disk acts as a full-fledged setup tool for upgrading other Macs to the new OS.
