|a consumer would have a reasonable expectation that a 1394 equipped storage device would inter-operate with a 1394 cable STB
There's a thread on the DVR10(?), that I'm too lazy to link in, that might shake your confidence in this assertion. Granted, the discussion there is about protocols that run over the the 1394 connection not being supported by certain hardware, but...
Separate from that, there's no requirement for Comcast to configure the software on the box to use the external storage, even if the device recognizes the new drive.
To be honest, I really
hope I'm wrong about this. I'm trying to get through my 40+ hour backlog of shows on my upgraded DirecTivo, and fully expect my save up and watch viewing habits to get me in trouble with a 7 hour DVR. (I'm configuring a PC PVR to take up some of the slack by recording all the SD content, leaving the [pending] 6208 free for HD-only recording)