How Hard Drives Work?

Hard drives have one, two, or more platters that stack together and spin in unison.

Read/write heads are controlled by an actuator and move in unison across the disk surfaces as the disks rotate on a spindle.

Figure below shows a hard drive with four platters:-

A hard drive with four platters.

1. All eight sides of these four platters are used to store data, although on some hard drives the top side of the first platter just holds information used to track data and manage the disk.

2. Each side (surface), of one hard drive platter is called a head. (Don’t confuse this with read-write head)

3. The four platters in the figure above has eight heads.

4. Each head is divided into tracks and sectors. Eight tracks (one on each head) make one cylinder.

5. As with floppy disks, data is written to a hard drive beginning at the outermost track.

6. The entire first cylinder is filled before the read/write heads move inward and begin filling the second cylinder.

7. A hard drive requires a controller board filled with ROM programming to instruct the read-write heads how, where and when to move across the platters and write and read data. 

8. Hard drive controller mounts on a circuit board or inside the drive housing and is an integral part of it. The controller and drive are permanently attached to one another.

Reference:-

A+ GUIDE TO Managing and Maintaining Your PC, by Jean Andrews