This is interesting, as I ran into a situation like this just tonight. I was changing my system from SWM back to a conventional multiswitch -- there are three DVRs connected: an HR20-700, HR21-200, and an HR24-500. Of these machines, only the HR21 gave me any trouble -- it kept reporting "X" on the odd channels of the 101 satellite.
I tried everything I could think of: changed cables, BBCs, even the multi-switch and the LNB. No change. Upon each "re-test," I would get error reports for different tuners -- it might say Tuner 1 had a problem, or Tuner 2, or BOTH. However, if I just ignored this error and moved along, I could find NO channels actually missing. Or at least, I checked about 50 SD channels, and they all came in.
I kept running the test (without changing anything else), and eventually it returned no errors and completed normally.
Perhaps this is some kind of bug in the 21-200?