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.
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.