Hard disks are rigid platters, composed of a substrate and a magnetic medium. The substrate – the platter’s base material – must be non-magnetic and capable of being machined to a smooth finish. It is made either of aluminum alloy or a mixture of glass and ceramic. To allow data storage, both sides of each platter are coated with a magnetic medium – formerly magnetic oxide, but now, almost exclusively, a layer of metal called a thin-film medium. This stores data in magnetic patterns, with each platter capable of storing a billion or so bits per square inch (bpsi) of platter surface.
Typically two, three or more platters are stacked on top of each other with a common spindle that turns the whole assembly at several thousand revolutions per minute.
There’s a gap between the platters, making room for magnetic read/write head, mounted on the end of an actuator arm.
This is so close to the platters that it’s only the rush of air pulled round by the rotation of the platters that keeps the head away from the surface of the disk – it flies a fraction of a millimetre above the disk. On early hard disk drives this distance was around 0.2mm. In modern-day drives this has been reduced to 0.07mm or less. A small particle of dirt could cause a head to crash, touching the disk and scraping off the magnetic coating. On IDE and SCSI drives the disk controller is part of the drive itself.
There’s a read/write head for each side of each platter, mounted on arms which can move them towards the central spindle or towards the edge.
The arms are moved by the head actuator, which contains a voice-coil – an electromagnet suspended between two powerful magnets. When current is applied to the electromagnet, it forces the arm to move.
The heads are designed to touch the platters when the disk stops spinning – that is, when the drive is powered off. During the spin-down period, the airflow diminishes until it stops completely, when the head lands gently on the platter surface – to a dedicated spot called the landing zone (LZ). The LZ is dedicated to providing a parking spot for the read/write heads, and never contains data.