I'm using a 1TB seagate backup plus external drive with my Mac Pro (2013) and it mounts fine yet refuses to read/write anything to it. (I have it formatted in ExFAT.) The weird thing is when I plug it into my USB 2 thunderbolt screen it works fine under both mac OS and Windows 8.1 using bootcamp. I can't for the life of me figure out why it won't work when plugged into the mac pro. I've looked for drivers finding none. I'd obviously prefer it connected as USB 3 than USB 2 so this is a big issue for me. If anyone has had the same issue with these or any other drives or has any idea of something else to try please let me know.