The Travelstar 5K1500 is a three-platter affair stuck in a 9.5mm case. While it might not be good match for thin and light notebooks, its three 500GB platters deliver a total of 1.5TB of storage, which is more than many users have in their desktops.
The really interesting bit isn't the capacity, though. This is the first three-platter 9.5mm design. HGST had to come up with some clever space saving tweaks in order to accomodate the third platter, while maintaining the same platter spacing used in two-platter drives. In fact, it uses the exact same platters found in the company's earlier 1TB models.
There are some downsides as well. First of all it is still too bulky for most notebooks. It won't be fast, either, as it is a 5400rpm unit and there are no plans for a 7200rpm version. Therefore HGST believes it will be used paried to a caching SSD drive that should help boost performance.