Common theme here - I bought a 4TB Duo specifically for Raid 1, and have been surpirsed at the long rebuild time. It progressed painfully slowly during initial data transfers (550GB), but sped up since that work is complete.
Not done, but looks like 5-6 days to rebuild. Initially dissappointed and concerned, I am satisfied despite the long rebuild because of features, ease of use, and price. From other posts, I do think WD should consider shipping the product Raid 1.
New buyers - apparently patience is the price for affordable at home Raid 1.