Sorry for the late reply, didn’t realise that replies had been posted. The drives are all WD Reds, I have 4 6TB drives installed. I’ve had the same error with a single disk reported again a few times, each time I put the disk in my PC, check SMART and do a check in try disk and nothing is reported. Put the disk back in, rediscover it and do a rebuild and it’s fine until the next time it flags another disk in error.