I'm using WD AV series hard drives. Internally I use a 1TB 2.5" one (there is no 2TB). On the external one I use 2TB also AV series, 3.5" in an Orico 3519SUS3 enclosure. They both work fine (the external one is new so I don't know yet how will go in time internal has already a few months). Probably any type of HD will work (for some time, but do you really want to replace your HD every year or so ?) but the difference in price is small so why not get an AV one ? Remember it's firmware it's optimized for video (sequential read/write), it's also low heat, low noise and designed to run 24/7 (check WD page).They are available on many online retailers.