So I bought a 4TB Seagate USB 3.0 drive that I plugged in yesterday. When I plugged it in, a windows popped up saying "the disk you have inserted is not readable by this computer". I checked Disk Utility and it was formatted in NTFS so I just reformat as Mac OS Extended, no biggie. Once that's done, it asks if I want to use it as a Time Machine backup, and I say no. Then I cloned my old 3TB drive onto the 4TB drive using Carbon Copy Cloner which took forEVER (like 24 hours because my machine only has USB 2.0). This evening, I pulled out the 4TB drive and put it into my machine. When I booted it up, it said "the disk you have inserted is not readable by this computer". I checked Disk Utility and it is formatted in Mac OS Extended but for some reason not readible. I tried to repair the disk, but it couldn't. I couldn't figure out the problem, so I had to reformat (again it asked if I want to use it as a Time Machine backup... no) then just restarted the cloning process again, this time with both drives plugged into SATA ports. What happened with pulling out the drive that made it unreadable? I've never had that happen before.