Major parts of Disk drives are made up of a variety of highly specialized parts and subassemblies that perform a narrowly defined function. These parts are:

  • Disk platters Read and Write Heads
  • Channel Read/Write
  • Arms and actuators
  • Drive spindle motor, servo control electronics
  • Buffer memory
  • Disk controller

We’ll now briefly discuss each subassembly.

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Disk Platters

A platter is the physical media that stores data on a disk drive. Disk platters can be described as rigid, thin circles which spin under the spindle motor of a drive spindle. Three layers make up platters:

  • The platter’s rigid shape is due to the substrate
  • The magnetic layer is where data are stored
  • Protective overcoat that reduces damage to the disk drive due to microscopically-sized dust particles

Substrates can be made from many materials, such as aluminum/magnesium alloys and glass. Because disk recording is microscopic and the heads are very close to the surface, it is important that they are extremely flat and inelastic to thermal expansion or contraction. They must also be nearly uniform in density and free of material defects, which could cause vibrations and friction (heat), when spinning at high speeds (rpm).

Disk drives are made by stacking multiple platters on top each other. The platters are then separated by spacers so that the disk heads and arms can access both sides of platters.

Write and Read Heads

These are the read/write heads, which are used to transmit data to and from the platter. The read/write heads record and play back data stored on magnetic layers of disk platters. They induce magnetic signals to be imprinted onto the magnetic molecules of the media. When reading, they detect these signals.

The distance between the platter’s surface and the beads is known as the flying height, or head gap. It is usually measured at around 15 nanometers for most modern drives. This distance is smaller than most microscopic dust particles. Given the extremely tight tolerances of head gap tolerances, it makes sense to provide a stable and clean environment for the thousands, hundreds, or even thousands of disk drives running in a server room. Although disk drives can be used in many environments, reliability numbers increase with air quality. This means that they will work in cool, dry conditions and are free from any airborne contaminants.

Read/Write Channel

Although we tend to view data in the digital realm only, physical recordings are analog signals. The digital logic’s 0s and 1s must be transformed into something that can be printed on magnetic media. Data on disk is not written language, but rather it expresses itself in the form of a magnetic signal on moving media. The subassembly of the disk drive that performs specialized digital/analog conversions is called the read/write channel.

The read/write channel can be implemented in small, high-speed integrated circuits which use sophisticated signal processing techniques. The read heads detect the magnetoresistive phenomenon very faintly and require significant amplification. It might be interesting for readers to consider how data is read from disk. This is because it does not depend on the magnetic signal written to media. It is actually done by measuring the difference in the electrical resistance of media due to different magnetic signals. Amazingly, the resistance can be detected by a very thin microscopically-shaped head that doesn’t make contact with the media, but floats above it at extremely high speeds.

Actuators and arms

The position of the read and write heads must be precise. Because heads are very small, disk arms are made of thin, rigid, triangular, lightweight alloys. The disk arms, like everything else in a disk drive are manufactured with microscopic precision. This allows the read/write heads to be placed next to the platters quickly.

The tile drive actuator connects the disk arms to the tile base. This is responsible for positioning them. Voice-coil drivers control the actuator’s movements. The name derives from voice coil technology that is used in audio speakers. Because some speakers vibrate at high frequencies in order to reproduce sounds, bow disk actuators can have voice coils that move quickly. You can sometimes hear the actuator moving back and forth, making clicking sounds.

Drive Spindle Motor, Servo Control Electronics

Drive platters are powered by the drive spindle motor. This is designed to keep the drive platters at constant speeds and minimize vibration for long periods of time.

The motor’s drive shaft is the actual spindle to which the platters are connected. The spindle is a little like an old 45-rpm record player’s inner core. However, the platters don’t drop over the core and are held in place. Separator rings are used to precisely space platters so that their surfaces can be accessed by disk arms and heads.

Buffer Memory

Disk drives’ mechanical limitations in reading and writing data on rotating platters limit their performance to about three orders of magnitude (1000 times) less than data transfers to memory chip. To speed up data transmissions between disk drives and storage controllers, they have buffer memory.

Disk Controller

All disk drives have an internal target controller that responds to commands from subsystem or host initiators. The storage controller is responsible for the execution of the command within a disk drive, in addition to communicating with the external initiator. The firmware is the software component of a disk controller. It is usually stored in eprom chips on the circuit board.