Seagate has a new hard drive technology / series called Shingled Magnetic Recording. It claims to increase capacity at a much lower cost / bit basis.
Here is a video that shows how it does it:
The drives are starting to ship in 8GB capacities. As noted in ExtremeTech:
"an 8TB drive for $260 is pretty impressive in terms of cost-per-gigabyte — 3.25 cents per gig, to be exact. As it stands, the cheapest 6TB drives on Amazon or Newegg are around $280 — or about 4.5 cents per gig."
However, these drives are spec'd at 5,900 RPM and an average read/write speed 150MB/sec and Seagate is positioning them for archive.
So are you interested in this for video surveillance?