Mechanical hard drives contain spinning platters that store data magnetically. To access the data, the drive has an arm with read/write heads at its tip. The arm glides above the surface of the platters to position these heads correctly, much like the lens in a CD player moves in order to focus its laser at the needed area. An actuator connects to the arm at an axis point, which it uses to control the arm's positioning.
Actuator Axis Functionality
When a computer needs to access data, it sends a signal to the hard drive's actuator to move the arm, and in turn the read/write heads, to the correct position. Using an electromagnetic coil, the actuator rotates the actuator axis, moving the connected arm. The arm does not reach across the entire surface of the platters; it only moves within the small range allowed by the axis point. The spinning of the platters allows access to otherwise out-of-reach sectors.
Other Types of Drives
Very old hard drives used a motor-based actuator instead of an electromagnetic coil. These drives had an actuator pivot that shifted to position the arm, rather than using an axis like modern drives. Solid state drives lack an actuator entirely, as they contain no moving parts, similar to USB drives and memory cards.
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