Secondary storage forms store program and data files while they are not being used.  Secondary storage media act as long term or permanent storage locations because they are not erased when system power is removed.  Information in most forms of secondary storage can be transferred to main memory or received from main memory so is a critical component to the computer system.

Magnetic Storage
Magnetic storage types are the most commonly used storage media and were first introduced in the second computer generation.  There are several main types of magnetic storage including magnetic tape, magnetic diskette, and magnetic hard disk.

Magnetic Tape
Magnetic tape storage was the main storage medium in the 1950s and 1960s.   This medium is made of plastic tape coated with magnetic material that can be magnetized to store data.  More recently, tape storage has found favor as a back-up or archiving tool.  Data stored on magnetic tape is stored sequentially.  Sequential storage means that data must be read from and stored to the tape in order.  This may be a limitation of tape storage as a real-time storage medium.  The amount of data stored on a tape depends on its density.  Common tape densities range from 800 bits per inch to 60,000 bits per inch.  These tapes can store 200 or more megabytes of data.

Magnetic Diskette
Diskettes were first introduced in the 1970s by IBM and have become a very popular, portable, inexpensive storage form.  Originally, diskettes were 8 inches in diameter and were flexible.  This characteristic earned the diskette the name floppy disk.  Currently, there are two main sizes of diskettes available.  Following the release of 8 inch diskettes, 5 1/4 inch diskettes were released.  Most 5 1/4 inch floppy diskettes available currently can hold 1.2 megabytes (million bytes) of information.  Today, 3 1/2 inch diskettes are more common.  There recording density is higher which allows them to store 1.4 megabytes of information in a smaller space.  Besides being smaller in size and density, 3 1/2 inch diskettes also are covered by a rigid plastic case which protects stored data.  Drives for this size diskettes are standard on most personal computers currently.  The plastic disk inside the diskette is spun by the drive at a rate of approximately 360 revolutions per minute.  Read/write heads inside the drive move back and forth across the diskette reading the magnetic polarity of data stored in areas on the disk.  Similarly, when data is stored to the diskette, the read/write heads are positioned at an available location on the disk and the magnetic material under the heads is polarized to represent data being stored.  The amount of time required to move the read/write over the correct location on the disk is known as Seek Time and the time needed to transfer data from the diskette to main memory is known as the Data Transfer Rate.  All types of diskettes are easily removed and transferred among machines.

Magnetic Hard Disk
Magnetic hard disk drives are larger, faster secondary storage devices that remain within one computer.  They provide the main storage location for system and program files used by the computer.  Hard disks are made of a rigid material like aluminum, ceramic, or glass and are coated with a material that can be polarized, similar to material used on magnetic diskettes. Inside a hard disk drive, there may be 2 or more disks (also know as platters) which can store data on both surfaces.   The disks, read/write heads, access arms and other needed components are sealed within a metal case to protect the hard disk drive from dirt and moisture.  Since the drive is sealed, more precise mechanisms inside the drive allow data to be stored at a very high recording density.  Thus hard disks can easily store 1 to 10 or more gigabytes (billion bytes) of data in a very small space (often no larger than 4 x 6 x 1 inches).  In addition to having high recording density, hard disks very low seek times and very fast data transfer rates.  The disks inside the hard disk drive may be rotated at rates of 3600-7200 revolutions per minute and may have seek time as low as 10-20 milliseconds.  A single computer can have multiple hard disk drives, each providing data storage space.