It is a storage device with very large capacity, which ranges from 1GB to Terabytes in the modern scenario. It is placed in side the cabinet of CPU (Although external hard drives are also available these days). It is not a removable drive in normal circumstances. A hard disk has a much larger storage capacity than a floppy disk. It is fitted inside the computer and cannot be seen by us.
There can be one or more hard disk in the system unit of a computer. They were originally called "fixed disks" or "Winchesters". They later became known as "hard disks" to distinguish them from "floppy disks." Hard disks have a hard platter that holds the magnetic medium, as opposed to the flexible plastic film found in tapes and floppies.
At the simplest level, a hard disk uses magnetic recording techniques just like the floppy disk. Major benefits of magnetic storage - the magnetic medium can be easily erased and rewritten, and it will "remember" the magnetic flux patterns stored onto the medium for many years. The working of a hard disk is similar to that of a floppy disk. In order to increase the amount of information the drive can store, most hard disks have multiple platters.
Hard Disk has four key components inside its casing.
1. The platter (for storing data)
2. The spindle (for spinning the platters)
3. The read/write arm (for reading and writing data)
4. The actuator (for controlling the actions of the read/write arm)
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Platters
The platters are the circular discs inside the hard drive where the 1s and 0s that make up your files are stored. Platters are made out of aluminum, glass or ceramic and have a magnetic surface in order to permanently store data. On larger hard drives, several platters are used to increase the overall capacity of the drive. Data is stored on the the platters in tracks, sectors and cylinders to keep it organized and easier to find.
The Spindle
The spindle keeps the platters in position and rotates them as required. The revolutions-per-minute rating determines how fast data can be written to and read from the hard drive. A typical internal desktop drive runs at 7,200 RPM, though faster and slower speeds are available. The spindle keeps the platters at a fixed distance apart from each other to enable the read/write arm to gain access.
The Read/Write Arm
The read/write arm controls the movement of the read/write heads, which do the actual reading and writing on the disk platters by converting the magnetic surface into an electric current. The arm makes sure the heads are in the right position based on the data that needs to be accessed or written; it's also known as the head arm or actuator arm. There is typically one read/write head for every platter side, which floats 3 to 20 millionths of an inch above the platter surface.
Actuator
The actuator or head actuator is a small motor that takes instructions from the drive's circuit board to control the movement of the read/write arm and supervise the transfer of data to and from the platters. It's responsible for ensuring the read/write heads are in exactly the right place at all times.
