The role of the read/write head is that of a transducer, converting electrical signals to magnetic signals when writing and from magnetic signals back to electrical signals when reading. The read/write heads are tiny electromagnets which converts the zeros and ones, signifying each bit, to and from the patterns of magnetic flux reversals recorded on the hard disk drive platters.
The read/write heads are often overlooked, but are probably the most sophisticated component in a hard disk drive. A failure of the read/write heads, or a head crash will require professional data recovery services in order to overcome the problem. Data recovery in these situations can be a complex process, particularly if the heads have crashed into the surface of the platters, as it will cause a lot of collateral damage to the magnetic recording layer.
Read/Write Head Types
The older conventional read/write heads, such as ferrite, metal-in-gap (MIG) and thin film, make use of the two main principles of electromagnetics. Firstly by applying electrical current through a coil a magnetic field is produced, used when writing the disk platter. Secondly, is that a magnetic field in close proximity to a coil will induced an electrical current to flow, which is used when reading from the disk platter.
Magnetoresistance (MR) and giant magnetoresistance (GMR) heads do not make use of the induced current in the coil for reading data. They rely on the principle of magnetoresistance, whereby the resistance of the material changes when subject to different magnetic fields. This allowed a much higher recording density leading to a sharp rise in the maximum capacity of hard disk drives. Two separate heads could be used each optimised for their purpose, unlike the conventional read/write heads where a compromise was required; this allowed a further increase in recording density.
Tunnelling magnetoresistance (TMR) whereby microscopic heating coils where used to control the shape of the transducer region of the head, which allowed a further increase in hard disk capacity. This was closely followed by a move to Perpendicular magnetic recording (PMR), in which the magnetic field is recorded perpendicular to the surface of the platter, resulting in a huge increase magnetic density, leading to a sharp rise to disk capacities in the terabyte range.
Read/Write Head Air Bearings
The read/write heads must fly at a constant “flying height” above the surface of the disk platters, to ensure that they will not impact with them, resulting in a head crash. This is achieved by through the use of an air bearing, where air is drawn through the head by the action of the spinning platters, causing an area of pressure allowing the read/write head to float on a cushion of air.
In spite of the “flying height” being decreased significantly as recording density has increased, the read/write heads are now less likely to crash into the disk surface. Most head crash events are now caused as a result of an impact, such as a dropped external USB drive or laptop.
Data Recovery and Read/Write Heads
Any failure of the read/write heads will require the drive to be rebuilt by a professional data recovery company, such as DiskEng. If a head crash has occurred the platters will have also suffered damage, and part of the recovery process requires the imaging process to avoid these areas where possible, as they will probably destroy the replacement read/write heads, and also cause further damage to occur.