How a hard drive writes and reads invisible magnetic bits

A hard drive becomes silent when its power is removed. The platters stop turning, the arm parks, and current no longer flows through the recording head. Yet a document may still be there when the machine starts again.

The drive does not keep the file alive with a hidden battery. It stores a pattern in a thin magnetic coating on rigid disks called platters. Writing changes the direction in which tiny groups of magnetic grains are magnetised. Reading senses the fields around that pattern and turns them back into an electrical signal. Motion, magnetism and signal processing must all agree before the computer receives useful data.

A spinning route to any record

Early computer storage often forced a machine to work through records in sequence. Magnetic tape is still useful, but reaching a distant item on a reel takes time. A disk lets an arm move to the required circular track while rotation brings the wanted sector under the head. That ability to approach data directly is called random access.

IBM's 305 RAMAC, introduced in 1956, was the first commercial computer system to use a random-access disk drive. Its Model 350 unit used 50 disks, each 24 inches across, to store five million six-bit characters, equivalent to about 3.75 megabytes. An actuator moved a pressurised-air-supported head between surfaces. The Computer History Museum documents the machine and its specifications.

The organising idea survived: rotate a magnetic surface, position a head and find the requested address electronically. IBM's RAMAC history records the early work.

The platter is a landscape of grains

The shiny surface is not one uniform magnet. Its recording layer contains many microscopic magnetic grains. Within a useful recorded region, a group of grains can hold a preferred magnetisation direction. That stable direction is the physical state that persists after power is removed.

Modern conventional drives use perpendicular magnetic recording, meaning that the preferred magnetisation points mainly through the disk surface rather than along it. A magnetically soft layer below the recording layer helps guide the write field back towards the head. NIST describes perpendicular media as small, nearly decoupled grains with their easy magnetisation axes directed along the disk normal. See NIST's account of perpendicular recording media.

Teaching diagrams show tidy arrows, but real grains are smaller and irregular. Boundaries, thermal stability, magnetic coupling and manufacturing variation limit reliable recording. Yale's materials guide explains the granular medium.

Writing: current makes a local field

The write element is a tiny electromagnet. Current through a coil creates a magnetic field in a shaped core. Near the disk, a narrow pole concentrates the field into a small part of the recording layer. Reversing the current reverses the field, allowing the head to switch the magnetisation of a passing region.

As the platter moves, the electronics create a sequence of directions along a track. The medium resists accidental switching, while the concentrated write field exceeds its switching threshold locally.

It is tempting to label every upward arrow “1” and every downward arrow “0”. That is only a teaching shortcut. Drives use recording codes, timing information and error-correcting data. The read channel interprets a pattern of magnetic changes; one arrow direction does not carry a universal meaning by itself.

Reading: a field changes resistance

Writing and reading are different jobs. A modern head places a sensitive read element close to the write element, but the read sensor does not need to force the medium into a new state. As the recorded pattern passes beneath it, small stray magnetic fields influence the sensor.

In a magnetoresistive sensor, electrical resistance changes with the magnetic state of very thin layers. Electronics hold a controlled electrical condition across the sensor and measure the resulting change. That converts a weak magnetic pattern into a changing electrical signal.

The discovery of giant magnetoresistance, or GMR, by Albert Fert and Peter Grünberg in 1988 enabled more sensitive read heads and earned the 2007 Nobel Prize in Physics. The Nobel committee described how a magnetic layer in a read head responds to the disk and changes the resistance of a layered structure. Read the Nobel Prize explanation. Later readers use tunnelling magnetoresistance, or TMR, in which electrons cross a very thin insulating barrier by quantum tunnelling. Seagate describes a current reader that uses TMR, a responsive free layer, a stable reference layer and an insulating barrier.

The analogue signal is not yet a file. The drive's read channel amplifies and filters it, recovers timing, recognises the coded pattern and checks error-correcting information. The controller then delivers a block of digital data to the computer. A hard drive therefore stores magnetisation, senses a waveform and reconstructs bits through rules known to both the writer and reader.

The head flies without wings

The head must pass extremely close to the platter because the field from a small recorded region weakens rapidly with distance. It cannot simply scrape along the surface. In an air-filled drive, the spinning disk drags a thin layer of air with it, and the shaped underside of the head's slider rides on that air bearing. Earlier IBM work established the flying-head approach that became standard. The Computer History Museum traces the development of flying heads.

The actuator moves the slider across the platter while servo information, reference patterns already recorded on the disk, helps the control system keep the head centred on the intended track. A particle, shock or loss of spacing can let the head contact the surface. That is the origin of the term head crash.

This choreography also distinguishes a hard drive from a solid-state drive. An SSD stores charge states in semiconductors, with no spinning platter or flying head. Both retain data without continuous power by different mechanisms.

Persistence is not permanence

Magnetic recording balances opposing requirements. The medium must resist random thermal changes so stored patterns last. It must still switch under the write head. The read sensor must detect a small field without disturbing it. The actuator must move quickly while keeping the head aligned and out of contact.

Smaller recorded regions can increase capacity, but they also produce weaker signals and become harder to keep stable. Engineering progress has therefore depended on better media, head geometry, sensors, positioning, coding and error correction together, not on shrinking one part alone.

The file that returns after a restart is the end of that chain. Electric current created a local field; the field changed a material state; the rotating disk preserved and presented a pattern; a resistance change became a waveform; and electronics rebuilt the bits. What looks like memory in software begins as a carefully controlled arrangement of invisible magnets.