Also, precise control by using one track as a servo was possible. This meant that a flying head could be used. A flying head is head aerodynamicly designed to used the air tunnel created by the spinning of the disk to fly above the tracks. This all means more dense disks. Also hard drives had multiple heads so time to move from track to track in a single cylinder was zero.
Optical disks have heads that move above the storage media and beam a laser down at it.
The first optical disks CD-ROMS were create by the manufacturer making physical pits in the surface of the disk. Then readers could translated these pits infinite times without wear. This new storage medium meant much more compact tracts and bits, since the beam could be focus tighter then a mechanical device could be. The final topic should be holograms. The idea is that a crystal can contain memory in 3D rather than 2D thus the information stored in a 1MM thick disk can contain 1000 times the information written on it CD style.