I may be wrong but I disagree with Neha Agrawal.
The parts of a disk drive are:
1. Power source
2. Platter (where data is written to/read from)
3. Read/write heads (performs actual write, for hard disks this is by a tiny magnet to change magnetic characteristics of a location to write or sense magnetic polarization to read; for optical write once it is a laser to burn the platter, pitting it with holes for writing and a laser/photomultiplier combination to detect the presence/absence of holes for a read; for rewritable optical media, the write head has a laser that can change some property on the surface of the media (most commonly the color) between two physical states (such as green and red color) and the read head consists again of a laser/photomultiplier combination that detects the presence or absence of the physical states. Also detects the start of a track of data and sends that information to the controller
4. Actuator moves the read/write heads into position from the edge of the disk to the closest to the middle of the disk that the actuator can safely reach. Actuator actually come in two types. Fixed that only read a single track or cylinder. These are often used on Enterprise level devices where fastest read time is required and are almost always positioned over the volume table of contents and/or data frequently read (though this purpose is becoming obsolete as on-disk cache size increases). The other is movable and this is the actuator that retrieves most data and is the type found on (to my knowledge) all consumer level products.
5. Motor provides the rotational movement to allow reading data across a sector, cluster, track, and/or cylinder.
6. Controller for the actuator:
a. Computes the physical address of the location for data to be written to/read from.
b. Using the information provided by the heads computes the rotational position for the specific cluster/block on the platter to be accessed (to write to or read from).
c. Signals the read/write head to read data from the cluster putting it into the buffer or write the data from the buffer onto the platters
d. Signals when data has been retrieved from the disk and is ready to be sent to the computer.
e. Signals when the buffer is ready to accept data from the computer
f. Signals when the buffer is full
7. Input/Output Buffer - where the computer sends data to be written to the disk and where the computer reads reads data retrieved from the disk.
A long time ago the computers running the processes were responsible for providing all the functions of the controller. It was actually possible back then to write a program to deliberately crash heads or break an actuator by either moving beyond the safe location towards the middle or pulling it completely off the platter while the platter was still spinning and then push it back so it might bump the edge of the platter. I had a friend do that. Just a minor error in his code. It cost like $40,000 to replace the heads, actuator, disk, and then unknown cost for loss of data on the disk.