The most common kinds of storage after memory are disks, either magnetic or optical. The advantage of either of these types of storage is that they offer enormous capacity for a very low cost.
Areal density: The number of bits that a disk drive can hold in a given space is called areal density. The first generation of drives used in microcomputers had an areal density of 15 megabits per square inch; current drives have about 5,000-10,000 megabits per square inch (5-10 Gigabits).
Magnetic storage, mostly in the form of hard disks, is increasing in density by some 80 percent a year, nearly equaling the rate of bloat in today's software. Advances come from new materials that coat both the surface of the disks ...