| This chart shows the progress of areal density over the last 43 years. The red line is drawn as a best-fit through the blue diamonds which are actual products. Key hard disk head technology developments are indicated. Note that the scale on left is logarithmic, not linear. |
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| A single hard disk usually consists of several platters. Each platter requires two read/write heads, one for each side. Each platter has the same number of tracks, and a track location that cuts across all platters is called a cylinder. All the read/write heads are attached to a single access arm so that they can not move independently. |
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| Each track is divided into sectors. High speed disks have an access time of 28 milliseconds or less, and low-speed disks run 65 milliseconds or more. The higher speed disks also transfer their data faster than the slower speed units. |
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A hard disk can store anywhere from few gigabytes to few terabytes such as Seagate’s 5 TB Barracuda desktop hard drive, whereas most floppies have a maximum storage capacity of several hundred megabytes.