You can pick up an oem 4TB hard drive for around $259, so why is it that you can buy a portable 4TB hard drive with usb 3 port for $189? It seems to me that at a price of $189 it would make sense to simply order a few of these drives for your nas and take them apart to get the 4TB hard drive out of the unit. I’m not sure why there is a $70 price difference between these two drives maybe the portable drive runs at 5400 rpm as apposed to a 7200 rpm.
Not really sure what the cause of the price difference is, but $189 is a pretty darn good price for a 4TB drive. If you need a lot of storage space at a little less then 5 cents a gb, the Seagate 4TB external drive is worth checking out.