Apologies for the silence. As advised, I contacted Etgear Support, who recommended I perform an OS reinstall with just the 3 working drives in it. This worked and all seemed to be working. The 4th drive was added and recognised - strangely after about 24 hours of being in the bay - formatted and synced with the RAID. This continued for about 2 days before the drive decided that it would disappear and the RAID is now reporting on the Admin page that it is "degraded". I'm unsure what to do now. I I know that the 4th drive is fine as I put it through a thorough surface scan before returning it to the NAS. its back with Support now as of this morning 😞
