A disk drive Mechanism A disk drive data on the tracks of a spinning disk surface and reads data from the surface by using one or more read/write heads on its access arms assembly. Most disk drives use a single read/write head for each disk surface. However, some faster systems use multiple heads on each access arm to service a number of adjacent tracks simultaneously.
As the access arms assembly moves in and out in the direction shown in the figure, read/write heads move horizontally across the surfaces of the disks. In the manner, the disk drive positions the read/write heads on any track on/from, which it wants to record/read data. In case of a disk pack, each usable surface has its own read/write head and all heads move together. Hence, the disk drive can simultaneously access information stored on the tracks constituting a cylinder through the disk pack. Recall the cylindrical storage arrangement of information in a disk.
Read/write heads are of flying type. They do not have direct contact with disk surfaces. There is a separation of about 0.00002 inch between a read/write head and its corresponding disk surface. This prevents wear of a disk surface. However, read/write heads fly so close to disk surface that if a dust particle (typically of 0.0015 inch size), smoke particle (typically of 00025 inch size), finger print (typically of 0.00062 inch size), or a human hair (typically of 0.003 inch size) is placed on the disk surface, it would bridge the gap between read/write head and disk surface, causing the head to crash. A head crash, in which the head touches the disk, destroys the data stored in the area of the crash and can destroy a read/write head as well.
An optical disk drive uses laser beam technology for reading/writing of data from/to an optical disk surface. It has two laser beam sources. It uses one laser beam (of greater intensity) to write data by etching microscopic pits on disk surface and another laser beam (of lesser intensity) to read data from light-sensitive disk surface. To write data, it turns the laser beam ON and OFF at a varying rate due to which tiny pits (visible only through a powerful microscope) are burnt into the metal coating of disk surface along its tracks. To read data, it focuses the less-powerful laser beam on disk surface. This beam is strongly reflected by the coated surface (known as a land) and weakly reflected by the burnt surface (known as a pit), producing patterns of ON and OFF reflections, which are converted into electronic signals of binary 1s and 0s by a sensor. It uses a mechanical forward-backward moving read/write access arm with laser source and detection lens (called eye) assembly to place the unit at the desired place on the optical disk. This assembly and the disk do not come in contact at all. The disk itself is on a spindle attached to a motor that rotates the disk during access.