I'm trying to do a complete fresh install of Mavericks on my iMac 2012. It has the "1tb fusion drive" I can not get the drive to erase, no matter what I do. I boot into a installer flash drive, and use disk utility to erase the drive...it starts unmounting...crashes and reboots. I boot into a USB drive with an OS on it, same thing. I try to just unmount it, and it says it's busy and to stop any thing using the drive...though there's nothing.. Googling suggested killing spotlight...tried it, didn't help. I try to use diskutil unmountDisk force "/Volumes/Macintosh Drive" and terminal crashes. Can't Repartion the drive to try in that direction... all options there are greyed out. Can anyone suggest a solution?