[QUOTE=FreqNasty_RiseS;2683755]I saw a Seagate 4TB external on Amazon recently for around $160.00 including shipping to Australia. If you want to buy from a local store, Officeworks has WD 4TB externals for around $195.00.[/QUOTE]
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.