PC Hardware

Lesson 2: Hard Disk Drives

Hard disk drives are mass storage devices. Virtually all of today's computers have at least one hard disk drive. The first hard disk drives were small in capacity, physically large, and expensive, by today's standards; they were about 4 inches tall, 5.25 inches wide, 8 inches long, and weighed almost 10 pounds. In 1981, IBM introduced the XT computer with a 10-MB hard drive, and new owners wondered what they would do with all that space. Today, a new hard disk drive can fit in your pocket and hold over 17 GB of data. In this lesson, we examine hard disk drives, from the early versions to today's mini-monsters.

After this lesson, you will be able to:
  • Explain the operation of a hard disk drive.

  • Define the different types of hard disk drives, including their advantages and disadvantages.

  • Partition a hard disk drive.

  • Troubleshoot hard disk drives.

Estimated lesson time: 45 minutes

Physical Characteristics

The first form of PC mass storage was the magnetic tape drive. Although tape proved a good medium for storing large amounts of data, it had some significant limitations. The typical cassette drive cartridge was easily damaged. Further, gaining access to the data was slow due to the way data is organized on tape, as a long stream of ones and zeroes, an arrangement known as "sequential." Because tapes were hundreds of feet long, users often had to run the entire length of the tape to find the data they were seeking. Although by providing random access (the ability to go directly to any point on the data surface), floppy disks are a major improvement, they are too slow and too limited in capacity for modern applications.

The original concept behind the hard disk drive was to provide a storage medium that held large amounts of data and allowed fast (random) access to that data. Data on a hard drive can be accessed directly, without requiring the user to start at the beginning and read everything until finding the data sought.

The first IBM hard disk drives came out in the late 1970s and early 1980s and were code-named "Winchester." The original design concept included two 30-MB units in one enclosure: 30-30 (hence "Winchester"). The PC-XT was the first personal computer to include a hard disk. They were called "fixed disks" because they were not removable. (Old mainframe computers had hard platters that were removable.) The Winchester technology is the ancestor of all PC fixed disks.

Hard disk drives are composed of several platters, matched to a collection of R/W heads and an actuator. Unlike floppy disk drives, a hard disk drive assembly is housed in a sealed case, which prevents contamination from the surrounding environment. Each case has a tiny aperture with an air filter. This allows the air pressure to be equalized between the interior and the exterior of the drive.

The platters are usually made of an aluminum alloy and have a thin magnetic-media coating on both sides. After coating, the platters are polished and given another thin coating of graphite for protection against mechanical damage caused by physical contact between the data heads and the platter surface.

The R/W heads "float on a cushion of air" above the platters, which spin at 3500 to 12,000 rpm. The distance (flying height) between the heads and the disk surface is less than the thickness of a fingerprint.