Hitachi has developed a prototype four layer Blu-Ray disc capable of holding 100GB of data.

While companies such as TDK and Panasonic have previously mooted 100GB discs, they have always needed a specially developed optical head in the player to read the disc.

Hitachi, however, claims to have used a standard Blu-Ray drive optical head that's only slightly modified to allow it to read and write data across the four 25GB layers. It's believed this will only require a firmware update to make existing drives compatible.
http://www.pcpro.co.uk/news/126871/h...uray-disc.html