After many hours on the phone with Directv Technical support, it has been conclusively determined that the am21 external tuner is not
compatible with hr23 dvr. Directv has eliminated use of external tuners with this model since it is anticipated that they will be able to eventually provide all OTA subchannels via satellite. There is no software patch planned to address this non compatibity issue. I have requested that they exchange my new HD dvr23 with an older model 21 so that I can use my the am21 tuner until that unspecified future date. I am in an area where no subchannels are carried via satellite and the local PBS channel is in standard def on satellite.
I posted a response to your post on the directTV forum.
First, we have not seen any evidence that the AM21 won't work with the HR23. The HR23 runs the same software as the other DVRs and should work OK with off-air. Since I don't have an HR23/AM21 combination I can't guarantee there are no problems but no-one has heard of any. Second, DirecTV almost certainly will not support the subchannels at least in the foreseeable future, maybe never. In fact they have specifically resisted any attempt to make the subchannels "must carry". There are still many smaller DMAs that don't have SD locals at all and I am sure DirecTV will provide those locals before they even consider delivering subchannels. In DFW, carrying all the subchannels would require more than twice the bandwidth currently allocated to SD locals and most of the subchannels are a complete waste. I bet some of them have daily viewing figures in the hundreds or less. And even if there were a plan to deliver some subchannels, the people you would have been talking to would certainly not be in a position to know this.
If it were not for the fact that I do not have an HR23 and an AM21 in my house to prove 100% that the combination works I would just say the whole thing is a load of BS from a tech who for some reason can't sort out your problem. IMHO you should post back here and discuss the problem you are having and we'll try to sort it out.
EDIT: Since you have a DVR and this thread is for receivers post back in the DVR subforum...