The difference is the itunes music is licensed and sold for you to keep. As I understand the betamax decision the court ruled it fair use only to time shift, not to keep recordings forever. The cable devices I've seen with external storage did not make it easy to use more than one external hard drive.
I am not planning on keeping recordings forever and no other sub should either. Yes the Itunes comparison is not perfect (just better than the rebroadcast one). I think that if you record LOST on a VCR, you OWN the tape not ABC and you can do with it what you want. Just like you can do the same thing with the same episode you buy from Itunes. No you can't sell it, rebroadcast it or other license violations, but you OWN both of them just the same.
Yes the talk right now is that you can move a HD from a receiver in one room to another and vice versa. And yea some may have more than one HD to swap out when one gets full. I hope this would be to get caught up on watching what they have recorded and not long term storage as anyone that has had a HD get corrupt or die on them would warn against.
How many people do you really think are going to add a 500 or 750 gig HD and then need more? Yea some of you are going to try and record every movie on HBO and then get pissed and rant when your HD dies, but most subs just need a little more space to get them through until the stuff they have recorded gets watched.
Let's not forget why the HD is tied to the account and not the device. It is for receiver replacements not permanent storage.