Head Stack Assembly
When the heads fail on a hard disk, the symptoms usually consist of clicking or ticking noises. More often than not, it is one or more physical read-write heads or the pre-amp circuit that have failed. In either of these cases there is a better than 90% chance of data recovery.
The function of the VCM and Heads
The VCM is the head positioning system and is controlled by components on the main circuit board. As each seek is made to read specific data on the drive, the processor on the circuit board calculates the exact position on the platters to read the data. The processor then sends the necessary commands to the VCM controller to get it to set the heads in position ready to read the data. The mechanics of this operation is;
The coil is mounted on the actuator assembly in close proximity to a strong permanent magnet (1 above). When current is applied to the coil, an electromagnetic field is created that forces the actuator arm, and therefore the heads to move to a position according to the amount of current applied. By precise control of the voltage/current, the heads can be moved to the exact position needed ready to read or write the data.