OTA broadcasts are less compressed than the same channels delivered via satellite, so as a consequence they will require more bandwidth from the disk. Now I think, but I don't know, that even a 5400rpm disk should be adequate for 3 streams (two tuners recording plus playback). Regardless, any issue with the disk will be manifest first on OTA recordings, so that is why I'm suspicious of your disk, or it's connection. USB 1.1 disks (if any exist) won't hack it. Very long or funky USB cables might also be at fault.
But it's hard to believe that all 3 of you have this same issue. So maybe the latest firmware broke something as you suggested above.