I just ordered up a 750GB WD My Book at Staples for $80. When I searched it on Amazon some of the reviewers said they were using them for ViP hook-ups with no problem. The sale at Staples ends tomorrow. (Interestingly the flyer said $90, but the price in-store was $10 less (?), plus tax of course.) They were out of them so I have to wait while one is shipped from another store (no extra cost to me) so I can't actually vouch for it yet, but I don't expect any problems. Some folks have had issues with My Books in general, not necessarily as an EHD. I figured for the price I'd give it a try...
Dish tech support confirmed for me today that 750GB is the upper limit, but not just for the 211. (I have a separate thread challenging that limit but no replies yet.)