WRITE_FPDMA_QUEUED error after adding two HDDs

I added two WD 4T Red HDDs, and the system immediately started reporting ADA2 errors (see attached). Smartctl tests failed with write errors. Amazon contacted WD and they said to replace the drive, so I installed a new drive and got same errors. System would not accept the new drive (dd I/O error) in the same Pool. So, I swapped bay3 and bay4 (both new drives) and rebuilt the pool and all seems well/healthy at this point. However, the ADA2 errors continue scrolling on the console and in /var/log/messages.