The 921 could indeed record from its ATSC tuner (probably wouldn't have gotten one if it didn't), but when doing so only 1 of the satellite tuners was available for recording. IIRC, I could watch OTA live while the sat tuners were both recording.
Was that before the digital switchover?. I remember being disgusted about not being able to record OTA. I'm on translators here and they didn't get digital running until just a short time before the switchover. In fact I traded for a 622 right after they came out because the hard drive in the 921 went by by and I traded up rather than get another hunk of junk!
So I guess I need to defer to your correction.