We don't have the external hard drive, but we do have the 722 dual receiver that can record up to two programs at the same time. So typically, we will have a dozen or more programs (movies and weekly series) and will watch them exclusively to be able to fast forward through the commercials. Annnnd, we also use the recorded programs whenever we are camped where our Trav'ler is blocked by trees/mountains/buildings...etc. Don't even have to have the antenna raised. So I suspect that a hard drive would operate the same way.
Ron