I'm not sure I understand your reply right...
But the 702/702T comes without the internal HDD, right? my guess is that it is completely the same device as the 722/722T, just without the internal HDD. Which then leads me to the assumption that you could just install a HDD yourself if needed?
What confuses me in your reply is that you say "no" to a potential upgrade, but yes to mirroring? wouldn't mirroring a recording include an internal HDD?
Cheers!
Thomas