A medium of data storage used by computers, particularly microcomputers of the late 1970's through the 1990's. Physically, a diskette consists of a circular piece of Mylar coated with magnetic material (similar to magnetic tape), encased within a plastic case that protects the fragile recording material (and, via a soft inner liner, keeps the material free of dust and other contaminants) while providing access for the disk drive's hub and read/write head. Early diskettes were encased in thin, flexible plastic jackets (hence the alternative term "floppy disks"); later diskettes used hard plastic cases with metal or plastic shutters to further protect the magnetic surface.

When inserted into a disk drive, the drive's hub rotates the disk inside its protective case, while the read/write head moves back and forth across the radius of the disk to point to one of many circular bands of data, or tracks, on the disk. (In modern disk drives, there are two heads, one for each side of the disk, that move in tandem; early disk drives used only one side.) Each track is divided into a number of sectors, running around the circumference of the track. Locating any given piece of information on a disk involves positioning the head over the right track and waiting for the right sector to rotate under the head; since these tracks and sectors can be accessed in arbitrary order, a disk drive and diskette are referred to as a "random-access device." The read/write head uses induction to read the magnetic patterns from the disk, or to write new ones to a specific location.

The amount of data that a diskette can store depends on the recording method used with it, and the number of tracks and sectors it contains. The most common disk format used on IBM-compatible machines (with operating systems such as MS Windows and Linux) uses 80 tracks per side of a 3-1/2" high density diskette, with 18 sectors per track, each sector containing 512 bytes, for a total of 1,440 kilobytes of information per diskette. Macintosh systems use a format with a variable number of sectors per track, allowing more information to fit on a diskette.

The earliest diskettes and disk drives were created by Al Shugart at IBM from 1967-69. They used magnetic disks that were 8 inches in diameter, and could store anywhere from 81 to 243 kilobytes of information. Shugart left IBM in 1969, and founded Shugart Associates in 1973, which worked to popularize the 8-inch diskettes. In 1976, Shugart introduced a smaller diskette, measuring 5-1/4 inches in diameter. (Legend has it that a Shugart representative was in a bar talking with a customer who wanted a smaller diskette size, and pointed to a cocktail napkin as being about the right size. This napkin was later measured and found to be 5-1/4 inches across.) The 5-1/4" disk would be adopted by the microcomputers of the early 1980's, including the IBM PC, Commodore 64, and Apple II.

In 1980, Sony introduced even smaller diskettes, measuring just 3-1/2 inches across. (They determined this size by measuring a standard men's shirt pocket.) The new diskettes featured more rigid cases with better protection for the magnetic disk inside. The first popular computer to adopt these diskettes was the Apple Macintosh in 1984; IBM-compatible machines, such as the IBM PS/2, soon adopted them as well. The high-density 3-1/2" floppy, storing just under 1.5 megabytes of information, continues to be a standard as of 2000.

With the ever-growing size of computer data files and programs, the venerable diskette has become less useful, and has been partially eclipsed by media such as CD-R and Iomega's Zip disks, not to mention the ease of using LANs or the Internet to transfer information, rather than physical media. Still, diskettes retain some usefulness in this day and age, particularly for emergency booting of a computer with a corrupted hard disk, as well as "sneakernet" information tranfer from one computer to another when no other alternative is available.

Source: Smart Computing article, "How Computers Work, Part II", August 2000 (available at http://www.smartcomputing.com/editorial/article.asp?article=articles%2Farchive%2Fr0403%2F07r03%2F07r03%2Easp)
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