If you are doing much data transfer USB2 is VERY SLOW. If you have a newer computer you are best with USB3, eSata or Firewire. Controller cards can be added to older machines for any of these. I'm a photographer and have over 70,000 pictures on my computer. I have an SSD for my "C:" drive and four 2TB internal drives. For backup I have a pair of external USB3 3TB external drives and a 2TB external eSata drive. Not to mention a bunch of older USB2 1 TB drives. For reliability you are best off to leave these units turned off when not in use, most have no cooling fans and heat is what kills drives.
Last, remember it is not a matter of if a hard drive fails, its when it fails. All drives will fail, its just a matter of when. So, whatever you get come up with a backup strategy and stick to it.