The hard disk is used as the internal storage for the PC. It is a mechanical device and therefore is a lot slower than other components of the PC such as the RAM. Nowadays a typical hard drive would be able to store up to 1TB worth of data. Although the higher end HDD are more towards 8TB (Terabytes).
Hard disk drives can come as internal, or they can also be bought as external drives. They have two connections on them, one being the molex cable from the power supply , the other being the data cable, interface type either being IDE (Integrated Drive Electronics) or SATA, Serial ATA (Advanced Technology Equipment) the date cable runs from the hard disk to the motherboard.
Capacity - How much data the disk can hold (e.g. 250 GigaBytes)
Interface type - The disks interface type, such as IDE, EIDE, ATA, SATA, or SCSI (Discussed below)
Data transfer rate - The speed at which the hard drive moves data from one point to another, the higher the better.
Spindle Speed - Given in RPM (revolutions per minute) "The spindle speed is the rotational frequency of the spindle of the machine". The higher the speed to quck data can be accessed but this is at the sacrafice of noise.
IDE stands for Integrated Drive Electronics.
SATA stands for Serial Advanced Technology Attachment, which is the successor of the old ATA drives. SATA is a more recent technology.
IDE disks also have jumper settings which are used to determine whether a drive is a primary or a slave drive.
The differences between SATA and IDE are:
SATA requires less power than IDE
SATA does not have any jumper settings to determine whether a drive is a Master or a Slave Drive.
SATA cables are smaller and more manageable allowing more space within the chassis for ventilation.
SATA technology also allows hot swapping, which means the drives can be removed/ replaced whilst the PC is powered on.
IDE and SATA are more commonly used on standard or desktop PC's.
An extension of IDE, is EIDE (Extended IDE). Whereas an IDE interface is only used for hard disk drives, EIDE can be used for other peripherals such as CD drives, tape drives, etc.
Another type of hard drive interface is SCSI (pronounced Scuzzy). SCSI stands for Small Computer System Interface.SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD drives.
SCSI is an intelligent, peripheral, buffered, peer to peer interface. It hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner. Up to 8 or 16 devices can be attached to a single bus.
With SCSI having the capability of being able to connect more than 8 devices to a single bus, they are commonly used in servers as they contain an array of disks and a tape drive which would all be able to connect to the same bus.
In your computer's hard drive, there's just a large shiny, circular "plate" of magnetic material called a platter, divided into billions of tiny areas. Each one of those areas can be independently magnetized (to store a 1) or demagnetized (to store a 0). Magnetism is used in computer storage because it goes on storing information even when the power is switched off. If you magnetize a nail, it stays magnetized until you demagnetize it. In much the same way, the computerized information (or data) stored in your PC hard drive or iPod stays there even when you switch the power off.
The platters are the most important parts of a hard drive. As the name suggests, they are disks made from a hard material such as glass or aluminum, which is coated with a thin layer of metal that can be magnetized or demagnetized. A small hard drive typically has only one platter, but each side of it has a magnetic coating. Bigger drives have a series of platters stacked on a central spindle, with a small gap in between them. The platters rotate at up to 10,000 revolutions per minute (rpm) so the read-write heads can access any part of them.
There are two read-write heads for each platter, one to read the top surface and one to read the bottom, so a hard drive that has five platters (say) would need ten separate read-write heads. The read-write heads are mounted on an electrically controlled arm that moves from the center of the drive to the outer edge and back again. To reduce wear and tear, they don't actually touch the platter: there's a layer of fluid or air between the head and the platter surface.
The most important thing about memory is not being able to store information but being able to find it later. Imagine storing a magnetized iron nail in a pile of 1.6 million million identical nails and you'll have some idea how much trouble your computer would get into if it didn't use a very methodical way of filing its information.
When your computer stores data on its hard drive, it doesn't just throw magnetized nails into a box, all jumbled up together. The data is stored in a very orderly pattern on each platter. Bits of data are arranged in concentric, circular paths called tracks. Each track is broken up into smaller areas called sectors. Part of the hard drive stores a map of sectors that have already been used up and others that are still free. (In Windows, this map is called the File Allocation Table or FAT.) When the computer wants to store new information, it takes a look at the map to find some free sectors. Then it instructs the read-write head to move across the platter to exactly the right location and store the data there. To read information, the same process runs in reverse.
How does an electronic computer manipulate all the mechanical nitty gritty in a hard drive? There is an interface (a connecting piece of equipment) between them called a controller. This is a small circuit that operates the actuators, selects specific tracks for reading and writing, and converts parallel streams of data going from the computer into serial streams of data being written to the disk (and vice versa). Controllers are either built into the disk drive's own circuit board or part of the computer's main board (motherboard).
With so much information stored in such a tiny amount of space, a hard drive is a remarkable piece of engineering. That brings benefits (such as being able to store 500 CDs on your iPod)—but drawbacks too. One of them is that hard drives can go wrong if they get dirt or dust inside them. A tiny piece of dust can make the read-write head bounce up and down, crashing into the platter and damaging its magnetic material. This is known as a disk crash (or head crash) and it can (though it doesn't always) cause the loss of all the information on a hard drive. A disk crash usually occurs out of the blue, without any warning. That's why you should always keep backup copies of your important documents and files, either on another hard drive, on a compact disc (CD) or DVD, or on a flash memory stick.