A mechanical hard drive has four main components that work together in the data-storing and recalling processes. Hard drives differ from system memory in that the devices are used to store data for a computer as opposed to running system processes. Magnetic drives are gradually being replaced by faster and more durable solid-state drives that don't feature moving parts; however, SSDs cost more for the same storage.

Disk Platter Storage

A hard drive's platters are the physical part of the hard drive responsible for storing data. Platters are circular, thin metal disks that have a diameter that's slightly smaller than the width of the device storage case. Modern hard drives can have more than one platter stacked on top of each other to expand storage capacity. Disc platters resemble optical discs with thicker metal and no protective plastic coating.

Spindle Controls Motion

The spindle is the part of the hard drive that's responsible for spinning the platters so the device's read and write arm can access and save data. Hard drive platters are stacked on top of each other on top on the spindle. The platters have a hole in the center for placement on the spindle and are held in place on the spindle itself with platter clamps. If the platters are not secured on the spindle, they may collide with other parts of the hard drive when moving and break.

Reading and Writing Heads

The read and write arm, also called the actuator arm, is the part of the hard drive that reads data already stored on the platter and writes new data on the platter. The actuator arm includes the read and write heads that float just microns away from the platter that perform the actual read and write tasks. The read and write heads physically read and record magnetic patterns stored on the platter.

Actuator Aligns the Heads

The actuator arm is connected to a part called the actuator that controls the positioning of the actuator arm relative to the disk platter. The actuator works with the spindle motor to position the actuator arm so it lines up with the platter to read and write data.

Other Parts Interface and Protect

Hard drives have other components that aren't central to the physical process of storing and accessing data. The other parts include the data connector, power connector, printed circuit board and device case. The data connector and printed circuit board are the parts that the computer uses for data requests from the hard drive whereas the the power connector handles the electrical current needed to run the device. The hard drive's case is an air-sealed component that protects the device and holds all the components in place.