How Things Work: Magnetic Storage Devices
In this so-called "Information Age," more and more of the data on which our civilization relies is stored electronically rather than on paper. The majority of electronic data-storage mechanisms are magnetic in nature. Compact discs and related technologies (such as DVDs) are the notable exception, as they use optical means of recording data. Hard disks, video tapes, audio tapes, most removable media (floppy disks, Zip disks, etc.), and even the stripes found on credit cards and your CMU ID store data magnetically.
Floppy disks and credit cards are made of different kinds of plastic, and hard disks are made of metal or glass, but they all have a coating of iron oxide particles on them. These particles are called ferromagnetic, meaning that they become instantly and permanently magnetized when exposed to a magnetic field. (Permanently in this case just means that they remain magnetized when the magnetic field goes away — they can be re-magnetized in the opposite direction by exposure to a different magnetic field.) You can think of the particles as tiny bar magnets, roughly half a micrometer (that's a millionth of a meter) long — one hundredth the width of a human hair. When they're magnetized with their north pole at one end, they represent a digital 0; when they're magnetized with the north pole at the other end, they represent a digital 1. Each 1 or 0 is called a bit, eight of which make up a byte, which in turn represents one letter or number. The capacity of a storage medium is the number of bytes of data that it can store. The capacity of hard drives is measured in gigabytes, or billions of bytes. The stripe on a credit card, on the other hand, can only hold 226 bytes.
The main operational difference between tapes and disks is that data can be written to or read from any location on a disk. Tapes are purely sequential devices, meaning that they have a beginning and an end, and if you want to read data in the middle of the tape you have to fast-forward through everything stored in the beginning of the tape. The data stored on a disk is generally organized into sectors and tracks. Unlike the continuous spiral of data on a CD or a vinyl record, the tracks on a magnetic disk are concentric circles. Sectors are wedge-shaped regions of the disk, akin to slices of pizza. Tracks and sectors form a polar coordinate system that the computer uses to keep track of which data has been written to which regions of a disk.
The heads of a disk drive, like the heads in a tape player, move across the surface of the storage medium to read and write data. Floppy drives, invented in 1967, use a noisy motor that moves the heads in and out between the center and the edge of the disk with a screw-type mechanism. The heads in a hard drive (introduced in 1956) are on the end of an arm that sweeps an arc across the surface of the disk. (Hard drives are generally made up of several stacked disks referred to as platters.) Once upon a time this arm was driven by a motor, but modern hard drives use an electromagnetic actuator not unlike that found in a stereo speaker. This mechanism, called a voice coil, can move the heads back and forth through their range of motion very quickly, and can also change direction much faster than a motor. The first hard drives to use voice coil actuators came out in 1986. You may remember that hard drives from the '80s made a burbly beeping noise when they were reading or writing data; that noise came from the special motor as it swept the heads back and forth across the disk.
Magnetic data storage media are so prevalent because they're inexpensive, can be made to retrieve data very quickly, and can be erased and re-written many times. The downside to using magnetic media for archival purposes is that the magnetic particles that store the data will eventually deteriorate, causing data loss. The lifespan of magnetically-encoded data is generally 10-20 years, whereas the optically-encoded data on a CD will last for centuries, arguably even longer than data printed on acid-free archival paper. If you have lots of important data stored on 5.25" floppy disks, now might be a good time to back them up to a CD.