Components used in HDD can be broadly classified into 4 categories - magnetic components, mechanical components, electro-mechanical components, and electronics. The magnetic components, i.e., the media and the head are the principal components that enables storage and retrieval of binary information. In HDD, information bits are stored in concentric data tracks on a rotating disk coated with magnetic media; the information is recorded as well as retrieved using the read/write head. Practical realization of such non-volatile storage and retrieval of binary bits, however, involves many other essential components, e.g., a motor to spin the disks, an actuator to make read/write head access the desired data etc. Figure 1.2 shows the important components found in a typical HDD. Functions and special features of some of these components are briefly explained here.

Head and Disk Data is recorded on a continuously spinning disk made of aluminum or glass and coated on both sides with a thin layer of magnetic material. The disk is mounted through a hole at the center on the shaft (spindle) of a motor that spins the disks. In desktop application, the disks are spun at 6,000 or 7,200 RPM. The spinning speed can be 10,000 RPM or beyond in high performance HDD. Disk is coated with several layers of other materials. Details of these layers can be found in any textbook on magnetic recording such as [36] and [202]. Two separate elements, the write and read heads, are used for writing data to or reading data from the disks. These two heads are fabricated together on a larger structure called the slider that serves several important purposes. The slider provides electrical connectivity to both heads, and helps to place the read and write heads in close proximity to the magnetized bits by flying over the surface of the spinning disk. Well defined aerodynamic surface is created on the surface of the slider facing the disk to achieve the desired flying characteristic. The air moving along with the spinning disk and entrained between the disk and the slider’s aerodynamic surface produces an air bearing that makes the slider float.
The surface of the disk must be very smooth to produce uniform readback signal from the heads flying few nanometers above the disk. However, smooth disk gives rise to a different kind of problem. As no air bearing is formed when the disk is stationary, the slider touches the disk surface and a stiction force is produced. Smoother the disk surface more is the stiction between the slider and the stationary disk. The stiction between sliders and disks opposes the applied torque during the spinning up of the spindle, and large starting current is required to overcome this torque. This problem was solved in the earlier drives by creating appropriate texture on a small annular ring, known as landing zone, on the disk near the center hole. The sliders are pushed to the landing zone before the spindle is spun down so that they rest on the textured surface when the drive is spun up next time. The area of the landing zone can not be used for data storage. An alternative method, the Dynamic Load/Unload, was later adopted to solve the problem of stiction between head and disk [7]. This method avoids contact between sliders and stationary disks by bringing the sliders out of the disk surface prior to spinning down. A lift tab extending from the arm engages a ramp structure as the actuator moves beyond the outer radius of the disk. The ramps lift (‘unload’) the heads from the disk surfaces as the actuator moves to the parking position. Starting and stopping of the spindle motor occurs only with the heads in this unloaded state. During spin up, the actuator arm is pushed over the ramp after the disk attains the specified speed so that the sliders fly. There is no need to reserve an area for landing zone when dynamic load/unload is used. However, a small ring near the spindle shaft is not used for storage of data due to many other factors such as limit of accessibility by the read/write head, EMI (electro-magnetic interference) generated by the motor coils etc. The innermost track on the disk surface used for storing data is known as the ID (inner diameter ) track (Figure 1.3). The OD (outer diameter ) track is created as close as possible to the edge of the disk. However, the magnetic coating in the region near the edge is often not as uniform as in the inner region. The quality of the magnetic layer must be taken into consideration while deciding the radius of the OD track.