I'd suggest running a HD benchmark tool to see what sort of transfer rate you're getting - this looks like there is something wrong, I would expect to see at least 90MB/sec on a slow drive (as long as it isn't SMR). 90MB/sec is 5.4GB/min, 324GB/hr, 7.77TB/day.And the answer is.......... 2.5 days.
The drive is fine. No SMART errors after formatting.