This is an odd one and I need some help. I have 2 WD20EARX drives that individually work fine (I put either drive into my drive enclosure and they work fine). But when I put both drives into the enclosure I can only see one drive. The drive I see is in the first occupied slot - I can swap the drives around and then I see the other drive because it is in the first slot. I can put them in slots 2and 4 (or any other combination you like) but I only see the drive in the lowest numbered slot.
Is there a problem with these drives that I need to know about??