Hitachi reseach and development is working on a new storage technology that is expected to increase storage density to 2.5 terabit per square inch. That's five times the capacity of the densest hard drives of today and thus could lead towards 10 TB HDDs. To achieve this density, Hitachi is developing a thermally-assisted recording method that heats up the media with a 20nm laser beam while the magnetic head is writing the smaller bits of data.
In thermally-assisted recording, magnetic grains can be made smaller while still resisting thermal fluctuations at room temperature. As the name suggests, thermally-assisted recording uses a laser to heat up the media while the magnetic head is writing the smaller bits of data. This enables the use of media that is magnetically stable at room temperature with the very small magnetic grains required for high-density storage.
Alas there is no concrete information on when the technology will be commercially available, and even if it is expensive they will have to find a successor to perpendicular magnetic recording, which according to the same source will have a hard time reaching even 1 terabit per square inch.