Here's an update for everyone in Phx.
My 8240HD internal drive gave up the ghost this weekend, so I went to my local Cox store (Scottsdale) to replace the box. I was given an 8642HDC box with the red "R" sticker on the back, (for the Rovi Passport software). I asked the clerk if it supported an external SATA drive and he said that it would work, but it wasn't officially supported by Cox. After reading Bryan_CoxPHX's post above, I wasn't too optimistic. I connected the box and let it boot without my external drive attached to see if the internal drive was only formatted for 160GB of the 500GB drive. I went to channel 1999 and looked at the DVR AVFS page. It showed 465GB available, a good sign. I then powered up my external 1TB SATA drive, and was asked the formatting question. After the format and restart, I see 1368GB available. So it looks like whatever issue that was preventing it from working before, as well as only using 160GB, has been removed. The versions of software are as follows:
Passport Echo Software: 3.1.124
Here's the interesting thing. The box clearly has an M-Series CableCard plugged into the back. I have the same type in my HDHomeRun Prime (which is awesome BTW). My Win 7 Media Center that's used with the HDHomeRun can see the Plus Package channels but the 8642HDC cannot. There must be a limitation on the Passport Software for the H.264 channels. I really have no reason to get WHDVR, as I only have one set, but it would be nice to see those channels without resorting to using WMC and my Xbox as an extender.
The biggest bummer was losing all my shows on the external drive. I had several hours of the Olympics in 3D. I don't think there's any way to get that again.