How Does Your Hard Disk Actually Work?

The hard disk is one of the key components of your PC. It is a huge storage device that holds programs and data in a quickly retrievable form.

Few components of your PC work quite as hard as your hard disk. From the moment you switch on your computer to the moment you turn it off, it toils tirelessly away, performing the vital tasks of fetching and carrying the data needed to run your software and load your files. Even when your PC is not being used, the hard disk is just as important, since for most users it is the computer's only large scale data storage device.

Hard disks have improved enormously in terms of speed and storage capacity since they were first introduced into PCs in the 1980s. However, the fundamentals of hard disk technology have changed little.

Hard disks store information on flat, circular disks called platters. These platters are rigid and have a magnetic coating on both sides in which the digital data is stored. They are called 'hard disks' in contrast to floppy disks, whose casing encloses a flexible platter.

Hard disk platters have a hole in the centre and are stacked on a motor driven spindle that spins them fast anywhere between 5,000 and 15,000 rpm, depending on the size and cost of the disk (faster is dearer). The higher the speed at which the platters spin, the faster data can be read from, or written to, the disk, the faster your programs and files can be loaded into memory or saved.

The job of reading data from the disk, or writing data to it, is taken care of by the read/write heads. Each platter has two of these devices, one above and one below. To store data on the hard disk, the read/write heads convert bits of digital data into magnetic pulses, and then reverse the process to read data from the disk.

The read/write heads are mounted on head arms, which are very thin, triangular pieces of metal extending over the surface of the platter. These head arms are in turn connected to an actuator that moves them over the surface of the platter to locate the data elements being sought.

The heads don't actually touch the surface of the platters when the disk is in operation, but ride over them on a cushion of air that is as little as 25 millionths of a centimeter thick. The delicate platters and arms are sealed inside an airtight case to protect them from dust particles, which are potentially dangerous to these precision built, fast moving items.

The operations of these components are controlled by a circuit board mounted inside the disk casing. This circuit board governs a number of things, including the speed at which the spindle motor turns and the actuator controlling the movements of the head arm.

The hard disk needs to link in with the rest of the PC's circuitry; hence, there's also a hard-disk controller acting as the interface to the rest of the PC system. With older hard disks, such controllers took the form of a card occupying one of the PC's expansion slots. Hard disk controllers are always integrated into the motherboard.

The data on the hard disk's platters is stored in a number of concentric circles, known as tracks. A track is divided into smaller areas called sectors, each one of which can store 512 bytes of data: a 40GB hard disk thus contains around 80 million sectors. However, if your data were stored in 512-byte chunks it would make the hard disk job of finding it, reassembling it and loading it into your PC's memory an extremely inefficient one.

Instead, the filing system used by Windows XP, known as NTFS, works in clusters, which are small groups of sectors. Typically, there are eight sectors in each cluster, each amounting to 4KB.

The sectors in a cluster occupy a continuous block of space on the hard disk, making it easier for the read/write heads to find these larger chunks of data in one sweep. The clusters themselves can be located anywhere on the surface of the platters; they are not continuous. In order to load a file, your hard disk has to find all the clusters of which it is comprised in the right order.

Windows keeps track of these clusters with the NTFS, marking each one with a unique number and then marking the final cluster in the sequence with an 'end-of-file' marker. When they are instructed to load another file, the hard disk's read/write heads move across the platters at very high speed, finding each cluster as they go along and loading the file into memory.

Hard disks have developed rapidly. In the late 2000s, an expensive PC might have a hard disk capacity of 120GB; now hard disks with 1,000 times that capacity are the norm. Not only are they bigger and faster, but they are cheaper. This is just as well, since for every rise in hard disk capacity there has been an equal increase in the size of the files that need to be stored for today's program versions.