Upload & Sell: Off
If you can reformat and then read & write, then I would surmise it's a file system error. If it is still inoperable following a reformat, you should have a discussion with the folks at Seagate.
I have 2 of the 3TB, but on USB 2 on my vintage iMac 2007 running SL.... I had some issues with making large copies and Time Machine until I removed ALL of Seagate's software.
Hoping to upgrade before too long.