The major parts inside a Hard Disk assembly include:

• Disk platter or platters.
• Spindle motor for rotating the platters.
• Electromagnetic read/write heads (one per magnetic surface).
• Access arms or armatures from which the heads are suspended.
• Actuator for moving the arms (with heads attached).
• Preamplifier circuitry to maximize read/write signals.
• Air filter and pressure vent.

Platters: Most platters or disks are made of an aluminum alloy, but ceramic or glass platters have also been used. The diameter of the Platters in Inches can be 21/2, 31/2 or 51/4, and the thickness of the media can be from less than 1mm to about 3mm. The platters are coated on both sides wit a magnetic material. Older drives used a ferrite compound applied by squirting a solution onto the surface and rotating the platter at high speeds to distribute the material by centrifugal force. This process left a rust colored ferrite layer which was then hardened, polished and coated with a lubricant. Newer drives apply the magnetic layer by plating a thin metal film onto the surface through galvanization or sputtering. These surfaces have a shiny chrome-like appearance.
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Spindle Motors: Most drives have several platters that are separated by disk spacers and clamped to a rotating spindle that turns the platters in unison. A direct drive, brushless spindle motor is built into the spindle or mounted directly below it. (Sometimes this motor is visible from outside of the sealed enclosure.) The spindle, and consequently the platters, are rotated at a constant speed. The first drives rotated at 3,600 RPM but over the years this speed has increased to 4800, 5400, 7,200 and now as fast as 10,000 RPM. In contrast, Floppy Disk Drives spin floppy disks at 300 or 360 RPM. The spindle motor receives control signals through a closed loop feedback system that stabilizes to a constant rotation speed. The control signals come from information written onto the platters during manufacture or with older drives, from physical sensors.
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Read/Write Heads: Both sides of each platter are coated to provide separate magnetic surfaces, and so there is one electromagnetic read/write head for each side of each platter. A drive with 4 platters would have 8 sides and 8 heads. Some drives use one side as a dedicated surface for control signals leaving an odd number (5,7,etc.) of heads for actual use. Each head is mounted onto the end of an access arm and these arms are moved in unison under the control of a single actuator mechanism. The spinning disk(s) create an air cushion over which the heads float. Depending on design, this air buffer ranges from 2 to 15 microns. By contrast, a smoke particle or finger print is about 30 microns in size!

The heads are not supposed to come into contact with the surface during rotation. Only when powered off should the heads come to rest on the surface, but this should be over a specific area of the surface, reserved for that purpose. Most drives built since the late 1980's employ an automatic parking feature which moves the heads to this designated region and may even lock the heads there until powered up.
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Head Actuators:The head actuator is the positioning mechanism used to move the arms and consequently the heads, back and forth over the surface. The first Hard drives used a Stepper Motor to position the heads. The stepper motor rotated in either direction by reacting to stepper pulses, and it moved the head assembly back and forth by means of a "rack and pinion" or by a band attached to the actuator arms. This design, still used for floppy drives, is not suitable for current Hard Drive track densities. The access speed of stepper motor drives was between 30 and 70 mSec.

For a long time now a Voice Coil actuator has been used to control the movement of the heads. A coil,attached to the head assemly, moves toward or away from a permanent magnet, controlled by the amount of current flowing through the coil. This is an analog system, with the exact amount of movement controlled by the exact amount of current applied. The actual position of the coil is determined by servo (or indexing)information, which is written to the drive by the manufacturer. The location of the heads over the tracks on the platters is adjusted to different tracks by reading and reacting to this information. The access speed of voice coil drives is between 10 and 20 mSec.
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Preamplifier circuitry: From inside the sealed assembly, there are electrical and control wires for the spindle and head actuator motors, and from the heads themselves. The flex cable from the heads usually has a preamplifier chip built in, inside the sealed assembly. This chip takes pulses from the heads (as close to the source as possible) and cleans up and amplifies these signals before transmission to the electronics outside of the housing.
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Air Filtering and Ventilation: Minor wear of internal components and occasional contact of the heads with the surface can cause microscopic particles to be loosened within the HDA. A permanent air filter is mounted within the air stream to remove these particles before they can cause damage to delicate mechanisms. Most drives also have a small vent to allow for minor air exchange from outside of the housing. This allows for equalization of air pressure so drives can be used in different environments without risk of imploding or exploding.
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