A magnetic head is a read/write sensor approximately the size if a grain of sugar and can be found at the end of an arm that traverses backwards and forwards inside the hard drive chassis to read and write magnetic information to all locations on the rotating disk.
Modern hard drives have multiple data storage surfaces, since each disk platter facilitates double-sided storage and higher capacity disks contain multiple disk platters. Each surface requires its own dedicated read/write head.
The MHA (Magnetic Head Assembly) is a series of heads (one per surface) which move in unison, so that all surfaces can be read or written to simultaneously.
The system is somewhat similar in principle to a vinyl record player, though unlike a record stylus, the magnetic head(s) do not come in contact with the rotating platter(s). Instead, they ‘float’ on a film of air generated by the rotation of the platter.
Any disturbance or shock to the disk while rotating can cause the heads to momentarily contact the rotating disk surfaces, resulting in damage to the heads and/or platter.
Particularly in the case of laptops, any impact such as leaning on the laptop casing close to the hard drive, or equally by way of experiencing a fall while in operation, the heads which should otherwise ‘float’ may come in contact with the platter surfaces momentarily. The resulting friction forces cause the disk motor to stop rotating, and the heads become stuck on the platter, where they should normally revert to a safer ‘parked’ position during normal disk shutdown.
In these cases, it may be possible to re-seat and re-calibrate the operation of the read/write system, although there is always a risk of head and platter damage.
‘Head-only’ damage is usually a recoverable condition, whereas platter damage usually represents an unrecoverable condition.
One major problem in the determination of platter damage lies in the fact that in a multi-platter drive, it is generally not possible to visually inspect both sides of each disk platter. Removing the platters individually will cause loss of rotational alignment of the platters as set during manufacture. Based on a number of other indicators, CDS will highlight cases where it is believed that a MHA replacement may result in a successful data recovery outcome. However, in these cases, there is always a risk that non-visible platter damage may damage the replacement MHA immediately, ultimately resulting in non-recoverability and an invoice for parts consumed