I will be buying a new iMac, debating on the 512 Flash or the 1TB Fusion with an external. The question is, what would be an ideal external. I have looked at this . . .Mercury Elite Pro Qx2 4TB. I was wondering if it could be set up as a Raid 1 on two of the disks and use the 3rd for TM back up and the 4th for extra storage.