It turns out I have been a bit hasty in condemning the Seagate 8TB drives.

Yes, the first one that died really was a bad drive.
But the replacement for it, as well as the other (original) Seagate 8TB drive, are both still fine.

They do both report the same strange S.M.A.R.T. values -- a sign of something strange, but given how little vendors document their stuff I'm not 100% sure those values really are "bad".

But the kicker for me was both the replacement, and other, drive misbehave like crazy in the backup array here, which has a JMicron JMB363 SATA port multiplier. That setup works flawlessly with all other drives I've used in it: WD Green 2,2.5,3.0,4.0TB drives, various Hitachi and Maxtor models, smaller capacity Seagate drives, and Seagate 4TB drives.

But put the Seagate 8TB drives in there, and the result is a series of semi-random SATA errors with no meaningful diagnostics. So today, I flipped the switch on the array to change the interface from eSATA to USB3. All is now fine again. Slightly slower as one would expect, but no more mysterious failures.

And given how funky JMicron stuff has been in the past, and their extreme reluctance to provide programming documentation for their chips, I'll have to score this one as a JMicron rather than Seagate issue.

Cheers
-ml