How Hard Disk Drives Work

Hard Disk Drives (HDDs) use spinning disks (platters) to magnetically store information.



Logic board

Platters (disks)

Read/Write heads


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Reading & writing data

Permanent magnets

Each permanent magnet (naturally magnetic material) has a "north" and a "south" pole where north poles attract south poles and vice versa.

Voice coil

Electricity circulated through the voice coil creates an electromagnet (a magnetic field produced by an electric current). Current flow direction through the coil changes the orientation of the north and south poles of the electromagnet, which moves the voice coil either toward or away from the north and south poles of the magnets. The intensity and duration of the current determine how quickly and how far the coil moves.

Voice coils move fast enough to pivot the arms from the outside to the inside of the platter (full stroke) over 50 times per second!


The ribbon transports all information and electrical current between the logic board and actuator.


The arms are connected and pivot together.

Read-write heads

platter surface


The disk-facing surface of the slider has specific shapes etched into it that manage air flow and pressurization. As the platters spin, an air pocket or “air bearing” is created and keeps the head ~2 nm (nanometer) from each platter – thinner than a finger print.

Read heads

The read heads are TMR (tunneling magnetoresistance) devices, which consist of an insulator sandwiched between two sensitive magnetic materials

Magnetic fields from the platter influence the closest magnet, causing electrons to tunnel or travel through the insulator and flip the polarity of the second magnet — thus, "reading" the platter's varying fields without disturbing them.

Write heads

Write heads create an electromagnetic field that positions the north pole of a domain either up or down. The magnetic field is such that one side of it is much more concentrated, while the other end is more spread out. This allows the field to influence only the atoms on one side of the field (the “right side” of the field in this graphic). A magnetic domain with a north pole up could be a 1 while one with a north pole down could be a 0. Each 1 or 0 is considered a "bit" of data.

Magnetic domain

Electrons in atoms create magnetic fields, and the direction they “spin” determines the direction of their north and south magnetic poles. Hard disk magnetic domains usually consist of about 100,000 atoms with magnetic poles oriented in the same direction.

Multi-platter writing process

Data is written on both sides of each platter in the same respective location; if something 16 bits in size is written to or read from the platters, 2 bits would be on each of the 8 platter sides to account for the total amount.

For example, the phrase HARD DISK could be written on both sides of four platters as shown in the graphic to the left.

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Platters & spindle

Cap & screws


Both sides of the platter can be used, with each side usually able to hold around 500 GB (gigabytes) of data for a total of 4,000 gigabytes (4 terabytes) between 4 platters.

Data storage comparison

If these bits (1's and 0's) were printed on 8.5x11 paper in a 12pt font, it would be 9.6 billion pages – enough pages to fill 957,000 standard four-drawer file cabinets. These file cabinets could completely cover 65 floors of the new One World Trade Center building!

Spacer rings

Spacer rings are placed between each platter, and are precisely milled to ensure the platters and arms align properly.

Fluid bearing

The spindle uses fluid bearings to limit friction, noise, and increase durability. The spindle shaft sits in a tight, airless, sealed chamber within the bearing housing surrounded by a thin layer of lubricant.

This lubricant fills the space around the shaft, and prevents the shaft from contacting the housing


The moving spindle has an attached permanent magnet which forms part of a basic electric motor with the stationary copper coils. Most hard drives spin the platters at 5,400 (90 hertz) or 7,200 (120 hertz) RPMs (Revolutions Per Minute).

Platter composition

Lubricant layer

A 1-2 nm thick layer of lubrication prevents friction between the protective layer and read / write heads.

Protective layer

A carbon based protective layer is applied, then covered in a 1 nm thick layer of lubrication to protect the media layer of the platter. The protective layer has a smoothness of at least 0.4 nm – like a perfect circle the size of the earth with only 5.7 cm (2.3 in) of variance or imperfection.

Media layer

The media layer is made of a magnetic material, usually an alloy of Cobalt and other metals, and is about 100 atoms thick.


The base of the platter is non-magnetic and is usually made from aluminum or glass (this layer would be many times thicker than those above if shown at full width).

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Hard drives have separate connections for the power and data cables, and a Jumper Block. The pins on the jumper block can be connected to slow data transfer rate to be compatible with older, slower hard drives.

Logic board

The logic board controls the read/write heads using a built-in map of the platters to determine which areas are available and which areas are occupied. It also controls voice coil positioning, spindle motor speed, overall power management, and the transfer of data to and from the hard drive.


A filter is placed at the edge of the platters to catch any debris created or disturbed by them. Some drives have a breath hole with an Activated Carbon filter that absorbs vapors, and prevents dust or debris from entering the drive. Newer helium drives are sealed and don’t have a breath hole.


Air is naturally circulated within the drive as the platters spin, while a breather hole allows the interior pressure to equalize with air pressure on the outside of the drive.


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