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.