A hard disk drive in a computer today will normally be a 3.5 inch or 2.5 inch drive, which is the size of the platters on which data is stored inside drives. Read/write heads move over the spinning platters to read and record data. Laptops will have a 2.5" drive and desktop systems will likely have a 3.5" drive, though may have a 2.5" drive. Before these sizes became common, there were 5.25" drives when Parallel ATA (PATA) drives were used. Seagate released the first 5.25" drive in 1980 and Rodime released the first 3.5" hard drive in 19831. Before 5.25" inch drives were used in desktop systems there were 8" drives. You can find some of the history of hard drive development in the Wikipedia article History of hard disk drives and the PCWorld article by Rex Farrance, Timeline: 50 Years of Hard Drives.
Hard disk drives have spinning platters within them, much as an old record player will spin a record, though the platter in a HDD spins much faster. Today you may find drives that spin at 5,200 revolutions per minute (RPM) or 7,200 revolutions per minute (RPM). What does that equate to in miles per hour (MPH)?
If a platter in the HDD is 3.5" in diameter then the radius is 1/2 that number, i.e. 1.75". A platter in a 2.5" drive has a radius of 1.25". The circumference of a circle is 2πr, i.e., 2 times the radius times the mathematical constant pi, which is approximately 3.14159265. So for a point on the outer edge of a 3.5" drive's platter that is spinning at 7200 RPM where a point on the outer edge of the platter travels 2 * π * r inches per revolution, the speed in MPH is approximately 75 MPH:
2 * 3.14 * 1.75 in/rev * 7200 rev/min * 60 minutes/hour / 12 inch/foot / 5280 foot/mile
For a 2" drive, the speed is approximately 53.5 MPH:
2 * 3.14 * 1.25 * 7200 * 60 / 12 / 5280
In kilometers per hour, those numbers equate to 120.7 kph for the 3.5" disk drive and and 86 kph for the 2.5" drive.
If a drive is spinning at the slower 5400 RPM, then the speed in MPH for a 3.5" drive is approximately 28 MPH (45 kph):
3.14 * 1.75 * 5400 * 60 / 12 / 5280 ≈ 28
For a 2.5" drive it is approximately 20 MPH (32 kph):
3.14 * 1.25 * 5400 * 60 / 12 / 5280 ≈ 20
References:
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Timeline: 50 Years of Hard Drives
By: Rex Farrance
PCWorld -
History of hard disk drives
Wikipedia, The Free Encyclopedia
