According to the WD forums, the EARS WD drives are not RAID compatible. This from the WD forum Community Manager:
Our only Green drives that are RAID compatible are Models: WD2002FYPS, WD1000FYPS, WD7500AYPS, and WD5000ABPS. None of the Caviar Green drives, including the EARS drives are RAID compatible. That’s why they will continue to fall out of the RAID arrays.
In Fact I’ve had three 2TB WD hard drives go bad on me in the last 6 months in my Drobo.