How to Erase Hard Drive Mac System (this partition can't be modified/unmount)? I want to erase a disk (Mac system) to resize OS X Mountain Lion to the max volume but I couldn't resize it nor unmount the other disk.
Turn off your computer and then turn it on again holding the "Option" key on your keyboard.

You'll be showed your boot options. You'll have to boot from a different disk than the one that you are trying to erase. Mac OS X won't erase the disk from which it is currently running.