Closely related to input and output technology is secondary storage, whereby a computer augments its memory capacity or stores the values of its memory in a non-volatile media. Non-volatile means that when the power goes off, the data in the memory are not erased, as is unfortunately the case with RAM memories. To avoid data loss during power failures, many computers have a battery backup which supplies electricity to the RAM if the regular power is cut.

In the infancy of computers, there was little difference between secondary storage and input or output. Humans could input data to a computer via punched cards, and the computer could punch out what was in its memory onto blank cards, thereby saving the data permanently, and also communicating with humans. But printers are not very good storage devices because the computer can't read the data back in once it is printed out on paper, although there are optical scanners that can sense the contrast of shadows and recreate the data from the printed page. Nowadays secondary storage is not used to tell humans what is in the computer's memory because the bits are microscopic in size.

Secondary storage trades off capacity for access time, or more succinctly time versus space (again). There is no faster way to store data than in transistors, i.e. flip-flops, but flip-flops are expensive and volatile. It also has a lower data density (fewer bits per unit volume) and runs hotter than the equivalent secondary storage. So the best strategy is to keep the data that are currently being worked on in RAM and put all other into secondary storage.

Secondary storage devices work by moving a physical medium past a read/write head, which is a device to pick up the data and convert it to electrical signals (a read operation), or to convert data from electrical signals to a physical encoding of some type and put it on the medium (a write operation.) Many devices use magnetism (tapes, hard disks, diskettes) while some use optics (CD-ROM). Each of these has its particular uses and drawbacks.

Probably the oldest surviving secondary storage method is magnetic tape, which we will not discuss in detail in this chapter since its method of operation is very similar to a disk drive. Today's tapes are much smaller and thinner than the giant wheels of old which defined in the public's imagination what a computer looked like. Blinking lights, which were never very useful, are also gone, making today's computers extremely boring! Only the flashy graphics on the monitor hint at the depths of complexity inside.

Tapes are still useful because a huge amount of data can be stored on a tape cartridge, making them ideal for backups. But they are very slow to use since accessing a random word of data from the tape might require rewinding the entire tape, a very time-consuming process. It is almost ludicrous that some of the earliest operating systems, called TOS (Tape Operating System), relied upon tapes for out-of-RAM storage.