So I opened one of the extra drives from a month ago or so, and the internal drives are a different kind of RED from what is sold online.
The model is WD80EFAX, so maybe it is of a lower grade or has different internal organs.
I checked the TS-831X logs and there have been no warnings for a few months. I think it was resolved by a QTS update in August or early September. There was nothing mentioned in the release notes though.
I was a bit surprised to note that the newest drives are listed as having been in operation for about as long as they have been installed. However, I have the power saving in effect and the drives spin down after 30 minutes of idle. Does a drive remain under power when parked and that time counts on the odometer so to speak?