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.