The largest HDD is 2TB, the HRs will not support a larger HDD.
I've got WD Green Caviar 2TB HDDs on/in ten HRs. They all work well, but the EVDS and EURS are both built specifically for DVR use. If you choose to believe WD. The WD Black Caviar drives will also work on the HRs, but are more expensive.
For an external device, I'd recommend the Thermaltake docking stations. Here's a link.
The WD drives seem to work with every HR as do the TT docking stations.