I am sorry to report it seems I had exactly the same issue. I added two compatible drives which were recognized but there was nothing to do in order to let the WD4000 perform the automatic migration from RAID 1 to RAID5 which should be one of the key features of this NAS. I made as well several reboots, I tried some hints suggested by the WD support without success. Pity especially considering how expansive and difficult to find are nowadays the drives compatible with the WD4000 which differs from the standard one only by a s/n code, i.e. you pay extra price without any additional feature except compatibility (but you can overcome this at least in a couple of ways).
As for many other issues of this NAS I had to find the solution myself but I am not sure if I can suggest it. My way out is described in an older post which I started when one of my two WD4000 did not perform the migration from RAID1 to RAID5 automatically:
Pleaes read carefully pros and cons. Maybe the easiest way for you could be just to backup data and perform a clean re-install. You will loose a few days for this but it might be the case that with a full re-install the WD4000 will maybe migrate the drives.
