For some weird reason, external hard drives, especially at the upper end of the market, tend to cost less than their internal equivalents although the former tend to carry a higher bill of material (enclosure, connector, cable etc).
Currys sells the 4TB Seagate Expansion desktop external hard drive for a mere £119.99, that's less than 3p per GB, not much more expensive than the cheapest GB on the market which currently stands at around 2.8p.
Top-of-the-range peripherals usually carry a significant premium but that's clearly not the case here. Currys for example sells the 2TB Expansion drive for £75 (which means that two of these would cost £150).
Externally, the drive adopts a very conservative design and looks like a brick, which is all the better as you will be able to stack it or place it either vertically or horizontally.
The Expansion drive has a spin speed of 5,400RPM and has a USB 3.0 port. It carries a two year manufacturer's guarantee and has a built in power management solution that automatically switches it off to save energy. Note that the total usable storage capacity is likely to be nearer to 3.7TB once the drive is formatted.