You could be having the TLER problem. WD green drives can take a long time to do error correction - this causes the raid to drop the drive as failed. This solution is to respond with an error quickly and let the raid fix it with parity.... but at least it doesn't drop the drive.
Or buy more expensive drives. Wait isnt that the idea behind RAID: Redundant Array of Inexpensive Disks..
You might want to look at this... http://hardforum.com/showthread.php?t=1285254
btw, I am currently working on recovering an 8 bay drobopro thats in a reboot loop...