Well I can now confirm the WD MY Book AV DVR expander works with Comcast's RNG200. No contact with the cable company was required. Unplugged my RNG200, attached the esata cable of the WD to the RNG, plugged in the power adapter of the WD and then plugged the RNG200 back on. After a few minutes, a screen comes up asking if you want to format the drive. (Yes or Cancel). The only problem is that which option is actually highlighted is not clear. I hit ok the first time, but my DVR was still 81% full, so I knew something was wrong although going through a list of my recordings was MUCH faster. Anyway I just unplugged my RNG200 again and waited for the format screen. Moved between both options to confirm that it's nearly impossible to tell which option you have actually selected, then did the opposite of last time. Voila, I'm now at 0% full and the RNG confirms it's using external storage.