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Hard Disk Drive Read/Write Head Stack

The read/write head stack acts as a transducer, converting magnetic signals recorded on the platters to electrical signals when reading the data and vice versa while writing. The read/write head contains tiny electromagnets to convert between the ones and zeroes and the data stored on the disk platters as magnetic flux reversals.

The read/write head stack is the most sophisticated component in a hard disk drive, playing an extremely important role in its correct operation. Any failure of the head stack or impact of the heads with the surface of the platters requires professional data recovery to recover the files. If the read/write heads impact with the platters, known as a head crash, the data recovery process can be very complex, especially if the magnetic recording layer has suffered a lot of damage.

Types of Read/Write Head

The earliest ferrite, metal-in-gap (MIG) and thin film read/write heads use the two main principles of electromagnetics. When electrical current flows through a coil it induces a magnetic field, used to write the data to the magnetic recording layer of the platter. The proximity of a magnetic field to a coil induces an electrical current to flow, used to read the data stored on the platters.

Later Magnetoresistance (MR) and Giant Magnetoresistance (GMR) heads use the magnetoresistance principle, in which the resistance of a material changes when placed within a magnetic field. By using heads based on this principle it was possible for the recording density to be increased, allowing much higher capacity hard disk drives to be manufactured. Conventional read/write heads used a single head, which by design were a compromise to handle both reading and writing data. By creating a pair of heads, each optimised for their individual purpose, it allowed a further increase in recording density was possible.

The next development in head technology was the Tunnelling Magnetoresistance (TMR) using microscopic heating coils which control the shape of the transducer region of the head, allowing another increase in magnetic recording density. Soon after Perpendicular Magnetic Recording (PMR) heads were introduced which record the data perpendicular to the surface of the platter, allowing the most significant increase in magnetic density, allowing hard disk capacities to reach a terabyte and larger.

Read/Write Head Air Bearings

Read/write heads must not come into contact with the platters and for the reliable operation need to maintain a constant “flying height” above the platter surface. Through the use of specially designed holes, called air bearings the spinning platters induce air to be drawn through the read/write head. This results in an area of pressure to build between the read/write heads and the platters, creating a cushion of air upon which the heads float.

Despite the increase in recording density and the much lower “flying height” used in modern drives, the read/write heads are much less likely to impact with the disk surface. Most head crashes seen in modern hard disk drive are now the result of impact damage due to accidental damage, such as dropping a laptop or an external USB drive.

Read/Write Head Failure and Data Recovery

The failure of the read/write head stack requires the drive to be dismantled and rebuilt, a process which should on be undertaken by a hardware data recovery specialist employed by a professional data recovery company such as DiskEng. A head crash is likely to cause damage to the platters which may result in the magnetic recording layer to be damaged. It is important that damaged areas of the platters are avoided as much as possible during the recovery process, as it is likely the replacement read/write heads be damaged, which is likely to cause further damage to occur.

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