Page 61 - UNI 101 Computer Science Handout.

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showing a disk platter, arm, and read/write head is shown in Figure below. This particular hard disk drive

contains three platters and six heads. Only the topmost platter and head are fully visible.

The entire assembly is sealed to prevent dirt particles from wedging between the heads and the disk

platter, since this situation could easily destroy the drive. Even a particle of cigarette smoke is much

larger than the space between the head and the disk. When the disk is stationary, the head rests in a

parked position on the edge of the drive. The head has an aerodynamic design, which causes it to rise

on a cushion of air when the disk platter spins.

Figure 4 A Hard Disk Mechanism.

Figure below shows the operation required to locate an individual block of data. First, the arm moves

the head from its present track until it is over the desired track. The time that is required to move from

one track to another is known as the seek time. Since the distance between the two tracks is obviously
a factor, the average seek time is used as a specification for the disk. Once the head is located over the

desired track, the read/write operation must wait for the disk to rotate to the beginning of the correct

sector.

61 Academic Year 2025/2026