I collected info from the failing drive in the array, and compared to other drives in the array.
It actually looked good, comparatively, until I found this one line near the end:
Warning: device does not support SCT Error Recovery Control command
GAH. No wonder. I still have to replace it.

This is another reason why Seagate is on my poop list.
They sent me a lower function device as a warranty replacement.

On the flip side, if anyone needs a 2TB 5900RPM drive for a desktop system, I can hoox you up.
It’s still in good condition, just not suitable for an array.
Also, it’s out of warranty (but only a year old).