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.
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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.
You are attempting to modify a disk that the system still wants access to, causing it to fail to unmount it.
Boot into the Recovery HD by holding ⌘R on boot, open Disk Utility and try from there.