A device that stores data on a rotating disk, or platter, coated with magnetic material. Disk drives are the most common means of secondary storage in modern computers. They were originally developed as a more compact and less expensive replacement for drum drives in the 1950s.
A basic disk drive consists of a flat disk, coated with magnetic material on both sides. A spindle attached to a motor extends through a hole in the center, and causes the disk to rotate constantly when the drive is in use. A pair of read/write heads, one for the top surface and one for the bottom, are mounted on a pivoting or sliding arm in order to move across the surface. The heads act like the heads in a tape recorder; they use a magnetic field to read and write data on the disk's magnetic coating. Larger drives may have more than one disk, stacked on the spindle with a head pair for each, and all of the heads mounted to a common positioning mechanism.
The disk drive's controlling electronics logically divide the disk concentrically into tracks or cylinders, and radially into sectors. The spacing of tracks is determined by how finely and accurately the head positioning mechanism can move the heads. Sector boundaries are usually referenced to a physical index mark on the spindle hub; from there timing of the rotation determines the sector boundaries. Conventionally, a sector of a given track contains 512 or 1024 data bytes, plus a small number of drive control and addressing bytes. The number of sectors, multiplied by the number of tracks and the number of disk surfaces, determines the disk drive's capacity.
Modern disk drives divide into two broad categories: "hard" or "fixed" disks (also sometimes referred to as "Winchester", after an old IBM drive model), and "floppy" disks. Hard disk drives have the disk platters, spindles, and head mechanism sealed inside a semi-airtight case at the factory; the air inside is carefully filtered. The disk platters are usually made of sheet aluminum, and the magnetic coating is highly polished. The spinning disk creates a thin layer of compressed air above it, on which the heads float, keeping the heads very close to the surface and allowing for high bit densities. Modern hard drives have very high storage capacities. Floppy drives use a disk contained in a small envelope or cartridge. The disk is made from plastic, and spins at a low speed compared to hard drives. The heads commonly operate in contact with the disk, which they can do because the rotating speed is low and the magnetic coating is fairly thick. The virtue of floppy drives is that the disk can be removed and a different disk inserted, and the cost of the disks is low compared to the cost of a hard drive. However, the storage capacity of a single disk is relatively low. Floppy disks have been used since the early 1980s for distribution of software, offline storage, and data exchange between users. However, the Internet and thumb drives have largely taken over these functions, and floppy drives are now fading from the scene.
From the 1950s until about 1990, "pack" drives were commonly seen. These were similar to hard drives in that they used disks of rigid aluminum, spun the disks at high speeds, and had high storage densities. However, unlike today's hard drives, the disks were removable. A disk pack consisted of a set of disk platters, mounted to a spindle, and contained in a pack enclosure that resembled a cake container. The most common type of drive contained a "well" where the pack went, with a top-opening lid or door which sealed the well when closed. To load a pack, the user removed the container bottom, opened the well door, and placed the pack (with the cover still on) in the well. The bottom of the spindle contained a drive plate which meshed with the spindle motor's drive plate at the bottom of the well, and located the pack precisely within the well. The user then turned a lever or pushed a button at the top of the pack cover, which allowed the cover to be withdrawn, leaving the bare pack within the well. The user then closed the door and switched the drive to RUN. The drive first purged the well with clean air, to eliminate any dust that may have gotten into the pack while it was stored. It then spun up the pack and extended the heads into the well to interact with the disks. Removing a pack was the reverse. The user switched the drive into LOAD; the drive withdrew the heads from the well and spun down the pack. After the pack stopped, the user opened the well door and placed the pack cover over the pack. The cover engaged the pack so that the cover's handle could be used to lift the pack out of the well. The user removed the covered pack and reinstalled the enclosure bottom. The pack could then be stored, or moved to another drive. Pack drives fell out of favor because the ability to remove the pack limited how accurately the disks could be located in the drive and how close the heads could fly to the disk surfaces. The technology reached its limits in the late 1980s, and as sealed hard disk drives improved, the price of the hard disks became competitive with the price of a pack with the same storage capacity. At that point, pack drives fell out of favor.
There were experiments in the 1970s with "fixed head" or "head per track" drives. As the name suggests, these drives had a set of heads for each track. They were very fast because the heads did not need to move; switching between tracks merely involved selecting a different set of heads. However, providing a set of heads for each track was very expensive. The technology had a brief heyday in the mid-'70s as a backing store for memory swapping with the era's very memory-limited systems; the falling price of semiconductor memory made it cost uncompetitive.