We’ve talked a few times of HAMR technology (Heat-Assisted Magnetic Recording), which allows you to achieve a much higher capacity in mechanical disks than we have today. TDK is working with this technology and says it can bring to 15TB HDDs in late 2015 / early 2016.
This technology consists in writing data magnetically on a disk using laser on preheated material for better performance. They make use of compounds of high magnetic stability such as iron and platinum alloy, which can store bits into a much smaller area without being limited by the super paramagnetic which limits the current technology used in disks today.
This improvement is achieved, as we say, by heating the material before applying the changes in the magnetic orientation ("Script writing" of bit).
Disk based on HAMR do not yet exist on the market, but for all we know Seagate plans to have 10TB disks for 2015 and 20TB in 2020 and like TDK that goes a step further and believes it will have the 15TB estimated for early 2016.