Officeworks had the WD external 4TB's for about $160 recently also.
Definitely prefer internal drives though, for my NAS. Unfortunately, externally powered USB drives often don't spin-down when the connected PC powers off, and often don't spin-down at all when idle.
Thats a big concern with 1) operational life and 2) power .. considering it'll be connected to my AMD-E350 NAS
HDD's chew up between 5-8W when spinning, and the entire NAS chews up about 15W at idle and only upto 35W at full tilt.