You need to use e-sata for the hard drive, USB is not enabled (though was briefly in test software a year plus ago, they never moved forward with USB hard drives).
I highly recommend the Wester Digital AV line, as they are designed for 24/7 DVR operation. I have a 3TB (WD30EURS) and have been very happy with it.
Most any e-sata enclosure should work ok. Bear in mind hooking up external disables the internal, it is not in addition to.
That in mind, and keeping more room on my desk (yes my HR44 is on my desk below the tv), I just replaced the internal drive on mine and 'sock drawer' the original in the event of a failure.
Antec makes a good e-sata enclosure, MX-1 if I remember correctly. Been a while since I've used one.