A simple introduction to the construction and operation of the Hard Disk Drive, an important component in many computer systems.
With the continuous growth in the availability of digital media, all this information must be stored somewhere. In the majority of personal computers, most files are stored on a hard disk drive.
What is a Hard Disk Drive?
A hard disk drive is a device used to store digital information. The hard disk was originally developed to store information for computer systems, but over the past decade or so the applications of hard disks have expanded to include many consumer devices (such as digital audio players and digital cameras). A hard disk is what is known as a non-volatile storage system; that is, the device retains data once the power to the device is removed.
How are Hard Disk Drives constructed?
A hard disk consists of a metallic case in which is contained one or more platters mounted on a spindle that allows the platters to rotate. The platters are made from a solid, non-magnetic material, normally aluminum or glass. The platters are coated in a very thin layer of a magnetic material upon which information is stored in binary form. For both sides of each platter, there is a read/write head mounted on an arm that moves across the surface of the platters.
The disk platters are spun at high speed that spins the air contained in the disk unit around with it. The read/write head is aerodynamically shaped and uses the movement of air over the platter to enable it to “float” at a very precise distance from the surface of the platter. This distance is very small, much smaller than the diameter of a human hair.
The construction of hard disks is performed in a cleanroom to prevent contamination of the disk surface by dust that may cause hard drive beeping. If the read/write head ever comes into contact with anything resting on the surface of the platter, or with the platter itself, very serious damage to the hard disk can result. For this reason, a working hard disk should never be opened once it has left the cleanroom. In order to prevent differences in external air pressure altering the shape of the hard disk, there is a small hole in the disk casing protected by a very fine filter to allow movement of air into and out of the disk drive.
How a Hard Disk Drive Works
As the platters spin, the magnetic read/write heads can write data by magnetizing minute areas of the magnetic surface and read data by detecting the magnetism of the previously written areas. The arms containing the read/write heads are moved using a voice coil that allows the heads to move over almost the entire surface of the platters. In order to read or write a particular piece of information on the platter, the arm moves to the appropriate position over the platter and waits for the rotation of the platter to bring the requested data under the read/write head. The movement of the arms over the surface of the platters is what makes the clicking noises that can be heard from some types of the hard disk.
Data is transmitted to and from the hard disk device by one of several different interface types. They are ATA (also known as IDE, EIDE or PATA), serial ATA (SATA), SCSI, Serial Attached SCSI (SAS) and Fibre Channel. The most common interface used in everyday PCs was ATA, but most modern PCs use the SATA interface. SCSI, SAS, and Fibre Channel are normally used in high-performance applications such as servers.
Applications of Hard Disk Drives
In a typical PC application, one or more hard disks are used to store information that is to be retained once the power to the PC is switched off. In most PC applications, the operating system (such as Windows, OSX or Linux) is stored on a hard disk. The PC is instructed to read the operating system from the disk by a boot program stored on a chip on the main circuit board of the computer.
Over the past several decades, the amount of data that can be stored on a hard disk has increased at an exponential rate. Today, a single hard disk drive can hold approximately one terabyte of data. One or more disks can be combined using the appropriate technology into a RAID array (RAID stands for Redundant Array of Inexpensive Disks) to create a single unit of a larger size, greater reliability, or higher performance, or any combination of the three.
The Future of Computer Storage
In some applications, particularly in the mobile computer and consumer device markets, solid-state disks, based on flash memory, are replacing hard disk drives. These devices, whilst more expensive than current hard disk units have several advantages. They use less power, and as they have no moving parts, data can be accessed much more quickly and they are more reliable.
Whilst other methods of storage are increasing in capacity and reducing in price, it will be a long time, if ever before they can match the low cost of data storage of a hard disk drive. Therefore, for the foreseeable future, any application that needs to store large amounts of data will utilize hard disk technology.