I have a 2013 Retina MBP. I was trying to do a migration during setup assistant and the MBP will absolutely not mount the 3TB (Seagate backup plus) drive containing the clone and time machine partitions. I have even booted into the recovery disk to see what was going on. It is (hopefully) not a USB issue as the MBP mounts a usb stick and another 2TB drive. My old (<2012) MBP can mount the 3TB in addition to the above. There are no disk errors. I saw one post on the seagate forum but thought it might be an anomaly (as it concerns booting rather than just mounting). Has anyone with a late 2013 MBP had this problem, or can tell me how to solve it? I have called Apple support and they were unable to solve the problem.