Does it possible to install Mountain Lion (clean install) without using USB key or DVD?
I'm currently running on SL, and I want install ML (I already have the dmg file).
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If you boot into the Recovery HD (⌘ cmd + R or ⌥ alt/option then select Recovery), you can open Disk Utility and Erase the internal drive.
Then close Disk Utility and use the Reinstall option in OS X Utilities to reinstall OS X.