Many types of hard disk drives are on the market, but nearly all share the same basic physical components. Some differences might exist in the quality of these components (and in the quality of the materials used to make them), but the operational characteristics of most drives are similar. The basic components of a typical hard disk drive are as follows:
- Disk platters
- Read/write heads
- Head actuator mechanism
- Spindle motor (inside platter hub)
- Logic board (controller or printed circuit board)
- Cables and connectors
- Configuration items (such as jumpers or switches)
Typical hard disk drive components.
The platters, spindle motor, heads, and head actuator mechanisms usually are contained in a sealed chamber called the head disk assembly (HDA). The HDA is usually treated as a single component; it is rarely opened. Other parts external to the drive’s HDA, such as the logic boards, bezel, and other configuration or mounting hardware, can be disassembled from the drive.
Hard Disk Platters (Disks)
A hard disk drive has one or more platters, or disks. Table below lists the hard disk drive form factors, the associated platter sizes that have been used in PC hard disk drives, and year they were first introduced:
Table Hard Disk Form Factors vs. Actual Platter Sizes |
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| Hard Disk Form Factor | Actual Platter Dia. (mm) | Actual Platter Dia.(in) | Year Introduced |
| 5.25-inch | 130 mm | 5.12 in | 1980 |
| 3.5-inch | 95 mm | 3.74 in | 1983 |
| 2.5-inch | 65 mm | 2.56 in | 1988 |
| 1.8-inch | 48 mm | 1.89 in | 1991 |
| 1-inch | 34 mm | 1.33 in | 1999 |
As you can see from the table, the dimensions used in describing the form factors are approximations; the actual platter diameters are usually somewhat different. Earlier in the article, I listed the exterior dimensions of drives using these various form factors.
Most hard disk drives have two or more platters, although some of the smaller drives used in portable systems have only one. The number of platters a drive can have is limited by the drive’s vertical physical size. The maximum number of platters I have seen in any 3.5-inch drive is 12; however, most drives have six or fewer.
Platters have traditionally been made from an aluminum/magnesium alloy, which provides both strength and light weight. However, manufacturers’ desire for higher and higher densities and smaller drives has led to the use of platters made of glass (or, more technically, a glass-ceramic composite). One such material, produced by the Dow Corning Corporation, is called MemCor. MemCor is made of a glass-ceramic composite that resists cracking better than pure glass. Glass platters offer greater rigidity than metal (because metal can be bent and glass cannot) and can therefore be machined to one-half the thickness of conventional aluminum diskssometimes less. Glass platters are also much more thermally stable than aluminum platters, which means they do not expand or contract very much with changes in temperature. Several hard disk drives made by companies such as IBM, Western Digital, Seagate, Toshiba, and Maxtor currently use glass or glass-ceramic platters. In fact, Hitachi Global Storage Technologies (Hitachi and IBM’s joint hard disk venture) is designing all new drives with only glass platters. For most other manufacturers as well, glass disks will probably replace the standard aluminum/magnesium substrate over the next few years.
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