If you want to make sure it's not the health of the drive which is at issue, you can download our free diagnostic tool SeaTools. In your situation, I'd probably run the Long Generic test to test all sectors of the drive. It takes longer, but will rule the health of the drive out by testing ALL sectors, the shorter tests may save on time but they choose sectors at random to test.
One thing you can try out to speed up the drive is to enable write caching:
Go to device manager > right-click external drive > click "Properties" > go to "policies" > click "better performance" > click "enable write caching.
Seagate Technology | Official Forums Team