Compared to main memory, secondary memory is slower, bigger, and cheaper (per unit storage.) Any method for secondary storage must involve two physical parts: a peripheral device (the component of the computer which 'reads' in or 'writes' out the information to/from the system unit,) and an input/output medium, on which the information is actually stored. Diskettes and cassettes are types of media. The medium has to be 'in' a corresponding peripheral device for information transfer to take place. In most methods of secondary storage, this transfer is realized by passing the medium by a read/write head, which is capable of sensing/writing information from/to that type of medium.

Secondary storage media may be removable (easily separable from the computer) like your diskettes, or fixed, like hard disks, which, for example, can be found in relatively expensive PC's. A fixed medium generally has much greater capacity (and is faster) compared to a removable one of the same type. On the other hand, by replacing your removable medium by another one when it is full, you can attain a virtually unlimited storage capacity. Another nice feature of removable media is that they allow backup copies of important material to be taken and kept away from the computer, so that they do not get corrupted if some disaster strikes the computer.

The act of retrieving pieces of stored information is called access. There may be two kinds of access to data stored as a sequence of items: Sequential access, where the items are traversed one by one from the beginning of the sequence to the desired one, and direct access (or random access), where any item can be accessed relatively independently of its location in the sequence. Different types of secondary memory media support one or more of these different methods, depending on their nature. Cassette tapes, for example, are sequential access media: You have to 'go through' the first 10 minutes if you want to listen to a piece which begins at the 11th minute of the tape.

The most common type of secondary storage medium is the magnetic disk. Diskettes (small disks made of flexible plastic) and hard disks (made of rigid metal) are the two different kinds of magnetic disks. Magnetic disks are coated with a magnetizable substance, the spots on which mean '0' or '1', depending on whether they are magnetized or not.

Each surface of the disk is subdivided into concentric rings called tracks. Disks with bigger capacity have more tracks than others. In larger computers, one stores the same amount of data in each track and keeps several disks mounted on a shaft on top of each other as a disk pack. The group of tracks which are at the same position in their respective disks on a disk pack is called the disk cylinder for that position. The associated peripheral device is called the disk unit. At least one read-write head is assigned for each surface. The read-write heads are mounted on a device called the access mechanism, which positions them on the cylinder in which the appropriate data item is to be located. The time for the read-write head to move from its initial location to the appropriate track is called seek time. In larger computers, the shaft on which the disk is mounted continuously rotates at a great speed, and the time required for the relevant portion of the track to come near the head is called rotational delay. The time needed for the actual information transfer between head and disk is called data movement time. Disk access time is the sum of these three components. In larger computers, the read/write head never touches the disk.

Diskettes, on the other hand, rotate only when a read/write is taking place. Furthermore, the head actually touches the diskette. Diskettes usually come in one of two diameters: 3.5 inches or 5.25 inches. 3.5-inch diskettes are products of a newer technology and can store more than 5.25-inch diskettes. Different types of computers employ different methods of data organization on a diskette. For this reason, a new diskette has to be formatted before it is used on a particular computer. Formatting a disk involves designating parts of each track as individually addressable (directly accessible) sectors. Each diskette contains a file directory, in which the names, lengths and starting addresses of the files (collections of related data) stored on it are listed. People with many files to store in the same disk are encouraged to use a tree-like directory structure in which related files are kept in separate subdirectories.

If a computer system has enough main memory and a program designed to read/write data from/to a disk is to be executed, one can transfer the contents of the disk to main memory and then use a method called disk emulation (or RAM disk) to 'trick' the computer into thinking that it is accessing the disk, while it actually deals with the much faster main memory.

An older type of secondary storage medium is magnetic tape. Characters of data are represented in byte form across the tracks which go along the length of the tape.

Other types of secondary storage media include mass storage units, which are combinations of spools of magnetic tape that can be automatically retrieved from a huge 'library' and read to disk. These store enormous amounts of information and do not require the intervention of a human operator, note that some disk packs and tapes need this kind of intervention.

Optical disks employ a relatively new technology. They use laser beams for the read/write operations. Most optical disks are of the CD-ROM (Compact Disk/Read-Only Memory) type; whose vendor-determined contents cannot be changed by the user. Because of their great capacity, these are used for storing multimedia (i.e. mixed text, graphics, sound, etc.) applications of great size. Their speed is low, compared to magnetic hard disks.