For it to be a valid failure, it needs to be an 8 drive system... since WD only say that their drives are certified for 8 drive systems
Great point. However, the older version of the same drive (WD60EFRX-68MYMN1), does not appear to have this problem. The WD Red Pro (Which I have ordered as a replacement) does not seem to have this limitation in it's description.
In my list of things to try, there is an experiment with a zpool of all "bad" drives. We'll see what happens.
I could also try to connect the bad drive directly to the motherboard, and place it somewhere vibration proof? However, this would also change these factors: Backplane not used, HBA not used.
BTW, how are your drives mounted? In trays? Are you using all 4 screws?
All drives are mounted in trays like these. All are mounted with 4 screws to the trays. 6 screws can be used. Maybe I should use all 6 mounting holes.