I have an iMac that is 4 years old and recently upgraded to a laptop. I would like to wipe the system clean and reinstall everything again (including the operating system). I booted into the recovery partition and attempted to erase "Macintosh HD" but it said the volume could not be unmounted. I know I could delete the user accounts; but I want it completely erased so it runs a bit faster. Is there any way to do this without the CD/DVD that came with it?