Fushigi
Storage Is My Life
Yes, the 722. Actually the external drive connects via USB. Brochure (PDF). Up to 750GB is in the notes (PDF again) although I wonder if that's an actual limit or if 750s were the largest available when it was written.The 722? I think it allows an additional hard drive to be attached via e.sata.
I'm guessing a 750 will at least double the recording time. But of course you can add as many drives as you want. they're used as archives and not as a playback source. You have to move saved shows to them & move them back to the DVR for playback.
There's a one-time $40 activation fee (per DVR, not per external drive) so I won't bother unless I find I need the space.