A hard disk drive, also known as a hard drive or HDD, is used to store data long term. Data can include the computer's operating system and applications as well as personal files including photographs, documents and music. When a computer is booted up, or turned on, the computer access the hard drive to load the operating system, such as Windows, OSX or Linux.
External or Internal Hard drives: All the hard drives that are inside a computer are called internal hard drives. External hard drives sit outside the computer in a case and they connect to the computer through USB, Fire wire or eSata. External drives are very useful for backing up your computer and for transferring files to two locations. External drives are also very portable and there are even smaller portable drives that use a computer hard drive that is very similar to a laptop hard drive. Hard Drive Capacity:

A computer hard drive is measured in Gigabytes. For example, a computer may have a 80Gb (Gigabyte) hard drive. Different files take more space. Video files and pictures take up a lot of room, but simple text files take next to nothing. The operating system and program files take up space as well.

Basic parts of hard drive: A hard drive has only a few basic parts. There are one or more shiny silver platters where information is stored magnetically, there's an arm mechanism that moves a tiny magnet called a read-write head back and forth over the platters to record or store information, and there's an electronic circuit to control everything and act as a link between the hard drive and the rest of your computer.

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  • 1. Actuator that moves the read-write arm. In older hard drives, the actuators were stepper motors. In most modern hard drives, voice coils are used instead. As their name suggests, these are simple electromagnets, working rather like the moving coils that make sounds in loudspeakers. They position the read-write arm more quickly, precisely, and reliably than stepper motors and are less sensitive to problems such as temperature variations.
  • 2. Read-write arm swings read-write head back and forth across platter.
  • 3. Central spindle allows platter to rotate at high speed.
  • 4. Magnetic platter stores information in binary form.
  • 5. Plug connections link hard drive to circuit board in personal computer.
  • 6. Read-write head is a tiny magnet on the end of the read-write arm.
  • 7. Circuit board on underside controls the flow of data to and from the platter.
  • 8. Flexible connector carries data from circuit board to read-write head and platter.
  • 9. Small spindle allows read-write arm to swing across platter.

Reading and writing data:
The most important thing about memory is not being able to store information but being able to find it later. 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.