A mechanical hard drive is based on one or multiple rotating platters, which store digital information in concentric lines (tracks). A good way to imagine how this works is the good old vinyl record or a CD. While records are based on physical dents in the surface and CDs or DVDs use optical technology (lasers) to detect those dents, hard drives utilize magnetism to differentiate between 0 and 1. Data can be stored by microscopically magnetizing small sections of a track in order to create a pattern of magnetized or non-magnetized sections along each track, which represent the stored information.
The units that read or write data by detecting the magnetic polarization or by magnetizing individual sections to set or erase a bit are called the "heads". You will find heads both on the upper and the lower side of a platter, as both sides typically are utilized to store data. Moveable arms similar to a vinyl record player’s tone arm position the heads above the desired track. Since the heads float on an air cushion and are extremely close to the surface once they leave the safe parking position, all mechanical drives are delicate. Hence you should avoid shocks or unnecessary movements of running hard drives. The most common reason for hard drive failure is a so-called head crash, which happens when the heads touch the surface.
Storage capacity of modern hard drives can either be increased by improving the recording technologies in an effort to increase data density or by adding more platters as long as they still fit into a drive’s form factor. Hard drive makers refer to data density either by talking about bits per square inch, or by providing information on how many gigabytes can be stored per platter. The last value, however, depends on platter diameter, so bits per square inch is non-ambiguous.
Platter diameter is another important aspect to look at: The larger the platter gets, the more data you can store on it. However, larger platters will require more head movement; hence increasing access times. And larger platters cause more noise and more friction, hence emitting more heat. This is the main reason why 5.25" hard drives died out: Their access time was too slow, and manufacturers were able to increase capacity significantly even when using smaller form factors.
There have been multiple generations of recording technology. The latest one is called perpendicular magnetic recording, which is referred to as "PMR". The vertical orientation of magnetized elements allows the hard drive makers to move the bits closer together. The old-fashioned longitudinal way of recording data is limited by the so-called super-paramagnetic effect, which is basically about magnetized elements influencing one another, which results in unwanted modification of stored data.
Perpendicular recording will remain in the future, but it will be improved by providing recording patterns to the recording media. This will be helpful to further increase data densities. A second option is heat assisted recording, which utilizes heat generating technology (e.g. lasers) to "unlock" magnetizeable sections before they can be physically modified. Both are necessary to sustain magnetism at increasing data densities. Hard drive makers are confident they can reach high double-digit Terabyte capacities based on the principles of a classic hard drives.
Each hard drive rotates at a spindle speed, which can be 3,600, 4,200, 5,400, 7,200, 10,000 or 15,000 RPM. You will find that 3.5" or 2.5" performance hard drives for the workstation or enterprise segments rotate at 15,000 or 10,000 RPM, enthusiast and desktop 3.5" hard drives spin at 10,000 to 5,400 RPM, 2.5" notebook hard drives are in between 7,200 RPM and 4,200 RPM and smaller drives in the 1.8" form factor rotate at 3,600 or 4,200 RPM. Even smaller drives such as 1" models by Hitachi and Seagate, rotate at 3,600 RPM.
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