YsarKain

Addendum to this....
 
The Toshiba drive was probably fine, but I RMA'd it anyways since it was under warranty.  Hopefully I hear back from Toshiba soon.
 
I continued to have issues and in the syslog I saw one of my Seagate drives was constantly giving "Power-on or Reset" errors.  Since I had managed to rebuild one of the drives I was able to try swapping drives around to try to diagnose the problem, and I think I did.
 
Turns out it was a power issue, at least partially related to the power disable "feature" in some newer drives.  The other Seagate drives in that enclosure would throw reset errors intermittently after I moved the first one I noticed having an issue, and when I tried swapping over some WD 10TB drives they wouldn't be seen at all.
 
I think the reason it did work silently for so long was that there are three power connections on the five bay enclosure.  The three connectors came from a 5-cable SATA power connector, and I had attached a splitter with two 4-cable connectors to it (so 2 ports did not have 3.3v, the third one did).  I unplugged one of the splitter connections and plugged in another connector on the original line that was close enough to plug in (with 3.3v) and haven't had a drive reset since.  What bugs me is that the opposite should be the case - I should have more issues with more cables that supply 3.3v, not less.  The alternative was to attach a sata-to-molex adapter and then a molex-to-sata adapter to that and then another splitter, which makes my skin crawl just to think of it.
 
The last drive is rebuilding now, been solid for a day and a half, and should be finished tomorrow.  I have two proper SAS expanders coming, finally decided to splurge, so my Frankenstein's Horror of four four-bay, one five-bay, and one three-bay drive cages, sitting on the floor, powered by a 1kw PSU sitting on top of it's box, hooked to a jumper board, with mini-SAS breakout cables and SATA power cables going every which way, will soon be replaced by two 15-bay, properly contained, towers 😁