Since the problem seems to be fixed with the Western Digital* hard drives, and based on the number of tests you just described, I'd recommend ensuring the Seagate* have the latest firmware updates. For this, please, contact the hard drive manufacturer.
Actually the firmware was one of the first things I checked. It was a very strange problem. The Seagate drives went in another system (different config) and worked fine, so I still don't know what the conflict was/is. Fortunately the Western Digital drives worked so I could move on to the next systems.