Hard drive storage capacities have changed dramatically since their creation in 1954 when IBM introduced the 305 RAMAC (Random Access Method of Accounting and Control) hard disk this would store about 8 MB. Although the basic components have not changed dramatically. There are seven basic components to a hard drives HDA (hard drive disk assembly), platters, the spindle and spindle motor, read/write heads, actuator, voice coil, and PCB.
The platters themselves come in several mediums. Traditionally platters are made up of an aluminum alloy that is coated with a magnetic material like ferrite compound in the form of a liquid that is applied to the platter as it is spun to allow an even coat. This compound is applied through an electroplating process. More recently with newer technologies manufactures have begun to use more heat resistant materials such as glass or ceramic platters. With theses materials drive manufactures have been able to place more platters within the hard drive cases allowing for lager storage capacities. Platters are coated on both the top and bottom a drive with 4 platters yields 8 sides in which it can store data.
The spindle and motor is what spins the platters at a set rate by the hard drive manufacture. Each platter is separated with a spacer that separates the platters at a set distance then is securely attached to the spindle. The motor is attached or mounted directly to the spindle which then rotates the platters in unison raging from 7,200 to 10,000 RPM.
The read/write heads magnetically writes data to the platters. The actuator arms hold the heads over each side of the platters. When a read / write command is executed the actuator arm holds the heads over the specified track with each head over it respective track surface. The distance between the head and the surface of the platter is less then a micron because of this a clean room Image is required during the assembly process. Typically when the platters stop moving the actuator arm moves to the inner most position of the platter. This area of the platter is non-magnetic to prevent the heads from sticking to the platters other wise known a head crash.
The heads are attached to the end of the actuator to allow them to move to their prospective area over the platter. Unlike older technology used to move the actuator called a stepper motor actuator. This type moved the heads based on a motor reacting to a stepper pulse. Each pulse would move the heads over the platter in predefined steps. This type of actuator would lose alignment easily and was prone to heat problems. A system known as a servo used in modern drive has evolved using a voice coil using a fluctuating current which controls the heads movements over the platters by moving a coil in-between two stationary magnets.
PCB or printed circuit board is the means for the hard drive to communicate with the computer.
The PCB holds the portions of the firmware and additives, controller for the spindle motor, as well as micro-code used to start-up the hard drive.