READ THIS GUIDE:
1. If you don’t have it yet, get Mountain Lion from the Mac App Store but do not install it yet (or redownload it if you did install it).
2. Create a bootable install drive for OS X Mountain Lion, make one manually with a USB drive or use the LionDiskMaker tool to automate the process with a USB or DVD
3. With the boot installer drive connected to the Mac, reboot and hold down the Option key.
4. Choose the “Mac OS X Installer” startup volume from the boot menu.
5. Select “Disk Utility” and choose the hard drive you wish to format, click the “Erase” tab, and then pull down the “Format” menu and select “Mac OS Extended (Journaled)” as the type, name the drive if you wish.
6. Click the “Erase” button and let the drive format — this is the point of no return
When finished, quit out of Disk Utility and now select the “Install Mac OS X” option from the menu.
7. Choose your freshly formatted hard drive and install Mountain Lion
When the Mac reboots you will have a clean installation of Mac OS X 10.8 to work with.
At this point you can either import files and apps from the backup you made, manually copy over backed up files, or just start anew.
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