I have one of WD's My Book "Elite" 1TB's on my 211k turning it into a DVR, haven't had a problem with the drive in 4 months (was initially a problem getting receiver authorized for DVR though)
I've a Seagate 2TB usb2/3 drive on my 722k and have had no problems with it so far either .. there is a slight lag on startup (didn't disable any sleep functions of the drive) but that's 1 to 5 seconds max, and while using I've had no problems during playback from it, or in moving recordings to it (can not currently record directly to the external drive on 722k)