How a Hard Disk Drive Stores Data on Spinning Platters
A hard disk drive records bits as tiny magnetic patterns on spinning platters. Here is how the heads, spindle and actuator work, and where it all began.
In this article
A hard disk drive, or HDD, stores data as magnetic patterns on fast-spinning disks called platters. A small read/write head floats just above each surface and either changes the magnetism beneath it or senses it. Because the patterns stay in place without power, the data survives when the computer is switched off.
The main parts
- Platters: rigid disks, usually aluminium alloy, glass or ceramic, coated with a thin magnetic layer
- Spindle motor: spins the platters at a steady speed
- Read/write heads: one per platter surface, mounted on a shared arm
- Actuator: a voice-coil motor that swings the arm across the surface
According to Wikipedia, the magnetic layer is typically 10 to 20 nanometres thick and is covered by a protective carbon layer. A data recovery firm's explainer lists the same basic pieces: several stacked coated platters, heads on an arm, and a spindle motor.
How a bit is stored
Data is stored as changes in the direction of magnetisation in the coating. To write, the head converts an electric current into a small magnetic field that sets the direction of a tiny region of the surface. To read, the head passes over those regions and its sensor reacts to them, producing electrical signals that the drive turns back into bits.
Modern drives use perpendicular recording, in which the magnetic regions point into the surface rather than along it. Wikipedia says this first shipped in 2005 and helps avoid a thermal instability called the superparamagnetic limit, where grains become so small that heat can flip them.
A head that flies
The head does not touch the platter. It rides on a thin cushion of air created by the spinning surface. Wikipedia notes that in modern drives the flying height is often tens of nanometres, whereas the IBM 1301 of 1962 flew about 6 micrometres above its disks. The drive is sealed against dust for this reason: a speck is huge compared with the gap.
Speed and layout
Spindle speeds range from 4,200 rpm in some portable drives to 15,000 rpm in high-performance server drives. Wikipedia says most consumer drives turned at 5,400 or 7,200 rpm as of November 2019. The surface is divided into circular tracks, and each track into sectors. To reach data, the actuator moves the head to the right track, then the drive waits for the right sector to rotate underneath. That waiting is why a drive's access time is measured in milliseconds, while memory chips and solid-state drives respond far faster.
The first hard drive
IBM's team led by Reynold Johnson invented the disk drive, and Wikipedia dates the invention to 24 December 1954. The IBM 350 disk storage unit was announced on 14 September 1956 as part of the IBM 305 RAMAC computer system. It used 50 disks, each 24 inches (610 mm) across, and held about 5 million characters. A typical record took around 600 milliseconds to find. The RAMAC system leased for 3,200 US dollars a month, and the 350 unit weighed over a ton, so it was moved with forklifts.
How capacity grew
Wikipedia states that areal density, the data packed into one square inch, has risen from about 2,000 bits to 1.4 terabits per square inch in 2023, and that the largest formatted capacity reached 36 terabytes as of 2025. Manufacturers count a terabyte as 1,000 gigabytes, while some operating systems count in powers of 1,024, so a new drive can seem smaller than its label.
Errors and failure
Drives expect some read errors. They use error-correcting codes to fix them and remap failing sectors to a spare pool. A monitoring system called S.M.A.R.T. counts these corrections, since many of them can signal that a drive is wearing out. Wikipedia cites uncorrected read error rates of about 1 in 10^14 bits for 2018 consumer SATA drives and 1 in 10^16 for 2013 enterprise SAS drives.
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