Western Digital has done it. They've made a traditional mechanical drive into one of the thinnest possible units imaginable. This drive is 5mm thin and they are offering it here at launch up to 500GB. This is a great solution for affordable ultrabooks, and it should be pretty dang popular. Take a look and see their press release.
WD has pioneered an entirely new way of engineering the hard drive. With the highest possible aerial density inside the thinnest space, the WD Blue ultra-slim hard drive is our most advanced form factor ever at just 5 mm thin. That's 35 percent thinner than most popular smartphones and 47 percent thinner than a standard mobile hard drive.
Source: Western Digital