Did you ever figure this out? After enabling spin down, two of my drives died - a ST8000VN04 dropped and also a WD 10TB Red completely died.
The ST8000VN04 doesn't have any SMART errors, and I was able to add it back to the array, but on array shutdown, I got a sector error in the logs (but nothingin SMART). Very weird.
Nov 5 20:02:07 nasserv kernel: XFS (md1): Unmounting Filesystem
Nov 5 20:02:07 nasserv kernel: md: disk22 read error, sector=3907717616
Nov 5 20:02:07 nasserv kernel: sd 1:0:6:0: Power-on or device reset occurred
Nov 5 20:02:07 nasserv kernel: md: disk22 write error, sector=3907717616
Nov 5 20:02:07 nasserv kernel: md: disk22 read error, sector=0
Nov 5 20:02:07 nasserv kernel: md: disk22 write error, sector=0
UPDATE... Looks like there maybe a compatiblity issue with the ST8000VN04 and Unraid?