I just made a Superduper (vers. 2.7.2) clone from my 3TB internal fusion drive. First I tried with a Seagate Backup plus USB3 3TB desktop edition. Superduper finished fine, but my iMac refused to boot from the disk. Then I tried with a WD My Book Desktop storage USB3 3TB, but same thing happened. Is there a problem with 3TB hard drives and booting on the late 2013 iMacs. The disks seem to work fine except from the booting problem. Disk Utility and Diskwarrior reports no problems, when I check for errors. What's going on?