They're dead, Dave
I woke-up to find an email from my NAS saying:
Storage pool 1 (SHR, ext4) on P3000 has degraded (total number of drives: 3; number of active drives: 2).
Information of the drives in abnormal status is shown below: Drive 10 Model: ST4000DM004-2CV104 Serial number: WFN2XYEG
Several reasons may result in storage pool degradation. Please go to Storage Manager > Storage Pool to understand the cause of degradation, or refer to this article to learn how to repair the storage pool.
That drive isn't even six months old.
When I check in the Storage Manager, it says that it has crash. Oh dear.
Off to PB Tech for a replacement, not swap the drive, add the drive back into the storage pool and … time passes (12 hours or so) … the new drive is integrated into the pool, and it all works. Pretty seamless and painless (except having to go out to get a new drive).
I love redundancy.