Inside a modern hard disk

Recording technology has changed too, with the first ‘perpendicular recording’ commercial drive appearing in 2005.

It took the form of a 1.8in drive from Toshiba, but Seagate was first to the market with 2.5in and 3.5in drives (both in 2006) using the technology.

It’s this technology, which allows for closer packing of bits on the magnetic media, that enabled Hitachi to launch the first terabyte (1TB) drive in 2007.

Last year also saw the advent of commercially available Flash-based solid state drives, or SSDs.

How do they work?
Hard drives work by storing and accessing data in a similar way to magnetic tape.

On a disk, the data is stored by changing the magnetic polarity of small portions of its surface, which is coated with grains of cobalt-platinum.

Think of a bar magnet ­ it can be pointing in one of two directions, with the north pole facing in the direction that disk is rotating, or against it. In the most recent drives, the direction of magnetisation is perpendicular, so the north pole is either facing towards or away from the surface of the platter.

The direction of magnetisation indicates whether a binary ‘1’ or a ‘0’ is stored in that bit; the disk heads either apply a stronger magnetic field to change the polarity of a bit on the surface, when writing data, or detect the current state to read it.

Each platter has a head moving over its upper surface, and all the heads in a drive are moved by the same actuator ­ effectively accessing a cylinder at a time, across all the platters. The heads have to be extremely close to the surface in order to read or write the data, around 0.0003mm.

To get that close to the disk surface without physically touching it, they ride on the air cushion created by the spinning discs. The only time the heads physically touch the discs is when the drive is switched off or if something drastic happens, say if the drive is dropped.

Most modern drives park the heads on a section of the disk reserved for that purpose, called the landing zone.

To read or write data, the controller has to wait until the appropriate sector passes underneath the disk head. The head position is monitored to check it’s correct, and any errors are fed back in a closed loop system, ensuring that the drive stays correctly aligned.

The heads themselves are built from magneto-resistive materials ­ this means their electrical resistance changes depending on magnetic fields near them, so as the disk moves underneath the head the resistance reflects the pattern of 0s and 1s stored. When data is written to the disk a current is applied to the heads, which creates a magnetic field that aligns the polarity of the part of the disk’s surface below the head.

Data is stored in sectors and tracks (put simply, tracks are concentric circles, which are divided along their length into sectors). Both of these are established by low-level formatting, when the starting and ending points of each sector are written.