Alternatively referred to as a hard disk drive and abbreviated as HD or HDD, the hard drive is the computer's main storage media device that permanently stores all data on the computer. The hard drive was first introduced on September 13, 1956 and consists of one or more hard drive platters inside of air sealed casing. Most computer hard drives are in an internal drive bay at the front of the computer and connect to the motherboard using either ATA, SCSI, or a SATA cable and power cable. Below, is an illustration of what the inside of a hard drive looks like for a desktop and laptop hard drive.
As can be seen in the above picture, the desktop hard drive has six components: the head actuator, read/write actuator arm, read/write head, spindle, and platter. On the back of a hard drive is a circuit board called the disk controller.
How is data read and stored on a hard drive?
Data sent to and from the hard drive is interpreted by the disk controller, which tells the hard drive what to do and how to move the components within the drive. When the operating system needs to read or write information, it examines the hard drives File Allocation Table (FAT) to determine file location and available areas. Once this has been determined, the disk controller instructs the actuator to move the read/write arm and align the read/write head. Because files are often scattered throughout the platter, the head will often need to move to several different locations to access all information.
All information stored on a traditional hard drive, like the above example, is done magnetically. After completing the above steps, if the computer needs to read information from the hard drive it would read the magnetic polarities on the platter. One side of the magnetic polarity is 0 and the other is 1, reading this as binary data the computer can understand what the data is on the platter. For the computer to write information to the platter, the read/write head aligns the magnetic polarities, writing 0's and 1's that can be read later.
External and Internal hard drives
Although most hard drives are internal hard drives, many users also use external hard drives to backup data on their computer and expand the total amount of space available to them. External drives are often stored in an enclosure that helps protect the drive and allow it to interface with the computer, usually over USB or eSATA. A great example of a backup external device that supports multiple hard drives is the Drobo.
External hard drives come in many shapes and sizes. Some are large, about the size of a book, while others are about the size of a cell phone. External hard drives can be very useful for backing up important data and taking with you on the go. They can store a lot of information, including music and movies.
Source : http://www.computerhope.com