The Basic Working Principle of Hard Disk Drives
Since their conception, hard drives have consisted of roughly the same core group of parts that provide both storage and retrieval functions.
Digital data comes in the form of ones and zeros that are inputted onto a hard disk by altering its magnetic polarity in various areas of its platter surface.
The Platter
Hard drives consist of multiple platters made from aluminium or glass coated with magnetic coating and covered in lubricant, while data is stored on tiny portions known as domains whose magnetism corresponds with either one or zero; the read/write head changes this magnetism when writing data into each domain, and platters spin at high speeds so read/write heads can move across their surfaces with remarkable speed.
As the read/write head passes over platters in read mode, its magnetic domains change polarization, creating weak pulse currents sensed by the head that is then translated back into operating system-readable data by electronic components inside of drive and controller assemblies.
The Basic Working Principle of Hard Disk Drives
Keep these points in mind about how hard drives operate: 1) All files on the hard drive are scattered about on its platter surfaces in an organized pattern of tracks and sectors. When an operating system needs access to one file in particular, its hard disk controller directs its read/write head where to look on the platter surface for that file on which platter; the read/write head then travels there using its servo mechanism (described further in Chapter 17).
Noting the hard drive platter spinning continuously for performance reasons. Since it is much heavier than the read/write head, moving it requires more time and power than simply turning it a small amount to reach new bit locations.
Hard drives are complex machines with numerous moving parts. Yet they remain astonishingly reliable. All movements and servo movements are controlled by an actuator arm connected to an amplifier; in turn, this actuator arm pushes against an actuator voice coil actuator (described in Chapter 18) for smooth performance that often lasts years without issue.
The Read-Write Head
The read-write head is a small electromagnet capable of converting magnetic flux into electric current or switching its polarity on the platter surface, representing either binary 1 or 0. It continuously scans its surface while spinning to detect differences in magnetism and decipher them as either 1 or 0, providing that information back to its computer host.
Underlying this system is the fact that individual grains of magnetic medium are organized into domains with similar magnetic fields. Within any one domain, magnetization lines from all the grains align in a certain direction to produce one single magnetic field at an individual level of the disk. When this magnetic field passes over one of its domains, its presence is detected by a head and voltage pulses are generated for every passage over that particular domain; when their amplitude exceeds a predefined threshold level, drive controller circuitry interprets this signal as logical 1 and sends it onward to computers for further processing and display purposes.
When an HDD is reading data, its head moves at 5,400 or more revolutions per minute across its platter. It consists of little more than a coil of copper wire, which generates electrical current when crossing its magnetic surface. The position of this head is determined by a servo motor controlled by computer commands to seek across its platter surface.
As soon as a read-write head makes contact with a platter, its electromagnetic field jumps across its gap. As metal conductors absorb this field more readily than air does, its intensity polarizes the storage medium it passes over so its magnetic particles line up with the direction of its magnetic field. Once written to, using its coil to alter specific areas on the platter where polarization needs to change is completed, and the write operation is complete. When reading back data again, however, this head acts like a flux transition detector, creating pulsed electrical currents when passing over the medium, which record flux transitions when passing over it again during the reading process – like its counterpart during the read process when the writing operation is complete.
The Arm
The arm is one of the largest and most complex human limbs, stretching from the shoulder joint to the fingertips and comprising of over 30 bones, blood vessels and muscles. Innervated by C5/C6 nerves as well as Radial nerves, arterial branches include brachial, Radial and Inferior Ulnar Collateral Arteries.
Hard disk drives are electro-mechanical data storage devices that rely on rigid, rapidly rotating platters coated with magnetic material to store and retrieve digital information. Each drive features an actuator arm to carry read/write heads across disk platters; an actuator controls these arms, typically called a voice coil motor; their bases sit between powerful magnets for stability, with movement controlled by passing current in and out of their coil to allow their movements and locate desired data bits more effectively.
Hard disk drives can store vast amounts of information even after computers have been shut off, using patterns of magnetic polarities on a disk drive to store that data; each domain pointing left represents one binary zero while those on the right represent one. Hard drives also contain circuitry to convert between user data and actual recorded signals for efficient functioning.
Hard drives contain all their moving parts in an enclosure that spins the platters and has an actuator arm with a read/write head on it, as well as a circuit board on its back that enables communication between itself and its host system.
A disk drive is powered by a motor that spins the platters at a fast rate to enable rapid head travel across them. These platters are securely mounted to a spindle that keeps them in their correct positions relative to an arm; electronics control rotation speed and proximity of the head proximity. Outer tracks have less densely packed sectors but still provide adequate capacity.
The Spindle
The spindle holds the platters in their fixed positions while rotating them at data transfer speed. It consists of a long screw spindle connected with suitable gearing to the cable drum and running inside a cylindrical housing, featuring a nut at its centre to support a thin disk made of wood from Euonymus alatus Rosa roxburghii (wedge-leaf euonymus).
Information stored on hard drives is encoded as changing magnetization patterns within small magnetic domains as the platters rotate beneath a head. Each tiny domain changes orientation relative to neighbouring domains as it changes orientation relative to one another; this creates patterns of north or south poles that correspond with particular bits in binary code. When writing operations are initiated, however, the read/write head activates magnetically polarizing bits on a platter in certain directions, and magnetically polarization of bits takes place; the resultant pattern represents digital data sent into hard drives from computers and storage media.
Operating systems access hard drives via an operating system that interprets their contents to determine where data files reside on the hard disk before checking their file allocation table, which stores all available storage space information about that hard disk, with two parameters as inputs to specify a location on that hard disk: radius from centre platter distance and angle relative to another point on a disk surface.
As the hard drive spins, its movable arm transports heads across each platter in order to access any location on it. To do this, it first looks at its file allocation table to see which areas of the hard disk are available for reading or writing before moving its actuator arm there and aligning read/write heads accordingly.
The head is an electromagnet that contains metal pieces encased by coils of wire connected by electrical current. When an active read-write head is activated, an electric current runs through this coil of wire and generates a magnetic field that attracts any bits that must be 1’s while bypassing those that need to be 0. When an HDD controller takes over data retrieval duties from its hard drives, it uses magnetic fields produced by these heads in order to decode data stored on them.
