A hard drive is a component in a computer system that is the primary drive and holds most of the data used on that computer system. Hard drives are sometimes called Hard Disks or as IBM refers, Fixed Disk Drives. The term Hard or Fixed comes from the type of platters contained in the hard drive which are rigid glass or aluminum and cannot bend or flex, also they cannot be removed like 3.5 floppy diskette drives. The operation of a hard disk drive is very similar to that of a floppy disk drive, in that both have a spinning disk that has floating heads that read and write data on to the disk.
Almost all disk drives in personal computer systems operate on magnetic principals, and use electromagnetism to store data. When electric current flows through a conductor a magnetic field is generated around the conductor, which in turn can influence magnetic material in the field. By applying alternating current (AC) the magnetic field's polarity can be reversed. Examples of how elecromagnetism is used are electric motors and electric generators. The read/write heads in your disk drives are u-shaped pieces of iron core wrapped with coils of wire which electric current can flow through. When the disk drives controller passes a current through these coils, a magnetic field is generated in the head. this magnetic field is formed in the area between the head and the surface of the disk, and the field's polarity is adjusted by the direction of the flow of electric current. Hard drive disks are aluminum or glass which a layer of magnetizable material is coated ontop. When the head is passed over the magnetic material on the platters the polarity of the material is designated by the current polarity of the drive head. The change in polarity of the alignment of magnetic particles on the disk surface is called Flux Reversal. A drive head places on flux reversals on a disk to record data. For each data Bit written a pattern of flux reversals is placed on a disk in certain areas known as bit cells. A bit cell is an area on the disk controlled by time and the rotational speed in which flux reversals are placed by the drive head. The encoding method is the particular pattern of flux reverals used to store a given data bit. Two types of encoding methods are Modified Frequency Modulation (MFM) and Run Length Limited (RLL), also all floffy disk drives use the MFM method of encoding. When a drive reads data off a disk, the drive head is passed over a disk surface in which a magnetic flux has been previously placed, a current is generated in the head coils and the direction of the current flow depends on the direction of the flux. When the head passes over an area where the flux direction reverses, the current flow in the head coil reverses also. So the currents generated in the head coils during read mode are identical to the origional current developed in the head coil to write the flux reversals, this data may be a weaker signal and is usually aplified so that it is the same as the origional data that was recorded.
There are may types of hard drives available today but almost all have the same basic origional components. The Head Disk Assembly (hda) contains the platters, spindle motor, heads, and head actuator mechanism and is a sealed unit. The Platters or disks are usually made of aluminum or glass and come in sizes of 5 1/4 or 3 1/2 inch diameters of the most common. Also it is common to have up to 15 platters in one hard drive. The platters are coated with a thin layer of Magnetic substance, and there are different types and methods of coating the platters. Oxide media has been used since 1955 and uses iron oxide particles. It is not used anymore on high capacity and high speed drives. Thin Film Media uses a thin film, thinner than oxide media and is used on the higher capacity drives, it is also known as plated or sputtered media because of the process used to plate the media. Plated media uses an electroplating method to plate the platter. Sputtered disks uses a process where aluminum platters are coated first with a layer of nickel phosphorus and then applying a cobalt alloy magnetic materal with a continous vacuum deposition process called sputtering. Plated and Sputtered media tend to be harder in composition and can withstand an impact of the head to the platter (head crash) more than oxide media.
Hard drives have a read/write head for each side of the platter, so there can be as many as 2 to 22 heads. Head mechanisms are basically simple. Each head has an arm that is spring loaded to put the head onto a platter and each platter has the heads above and below squeezed onto it. When the drive spins up to full speed, the heads lift away from the platter by air pressure developed under the heads. The head gap or distance between the head and platter at full rpms is usually between 5 and 20 millionths of an inch, this is why all head disk assemblies are sealed and never opened. If a piece of dust got inside th HDA the head could hit it and crash into the platter. Hard drives are assembled in clean rooms of a class 100 or better, where a cubic foot of air cannot contain more than .5 micron particles. There are three types of heads in hard drives, Ferrite heads, Thin Film heads, and Magneto-resistive heads. Ferrite heads have an iron oxide core wrapped with coils, they are the most common, largest and cheapest to produce. Composite ferrite heads have a smaller ferrite core bonded with glass and in a ceramic housing and are an improvement on the origional ferrite core. Thin Film Heads are made just like semiconductor chips and use a method of sputtering iron and nickel on hard aluminum. They allow a very narrow head gap adn are four times more magnetic than Ferrite heads. Magneto-resistive Heads are actually 2 heads developed into one and was pioneered by IBM. They have a standard thin film head for writing and a special magneto-resistive head for reading. Having two heads also allows each head to be optimized to its task. Mr heads operate as follows, the resistance of a current carrying magnetic conductor changes when an external magnetic field is present. Normal heads produce a voltage when detecting magnetic field flux reversal. MR heads sense a flux reversal and changes resistance. A small current flows thru the heads and the change in resistance is measured by this sense current. MR heads are three to four times more powerful than a thin film head during a read.

Besides the heads there is the Head Actuator Mechanism, which moves the heads across the disk and positions them over the desired cylinder. There are 2 basic types of head actuators, stepper motor type and voice coil type. The Stepper motor type of head actuator is generally slow and very much less reliable. Also it has a slow access rating is temperature sensitive and requires biannual reformats to realign the sector data with the sector header information due to mistracking. Actually stepper motor drives are down right inferior to voice coil actuators, also most older hard drives may have stepper motor actuators, especially if they require head parking. A stepper motor actuator operates with a stepper motor that moves from position to position by stepping with mechanical detents. Voice coil actuators operates by electro magnetic force. A coil is connected directly to the head rack, when it is energized it attracts or repels the head rack moving it in either direction. To determine what position the heads are at a system was developed that one side of a platter in the hard drive was designated when it was designed to be a track positioning platter. On it a special set of marks were prerecorded that indicate the proper track. Also the head on this track is only a read head so the marks cannot be erased. This system is called a DEDICATED SURFACE, CLOSED LOOP, SERVO-CONTROLLED MECHANISM. Voice coil actuaters are accurate and fast, also the auto park the hard drive.
Another part of a hard drive is the SPINDLE MOTOR. The spindle motor is connected directly to the spindle which is connected directly to the platters, gears or belts are never used. All Spinle Motors must be electronically noise free or the pulses created by the motor may cause improper data. Also the motor on most hard drives spins the platters at exactly 3600 RPM's, this speed must also be exact. Also most drives have no adjustment for rotational speed.
A hard drives LOGIC BOARD usually is in the bottom of the drive. This board will control the drive's spindle and head actuator. It also translates the data from the hard drive to the hard drive controller, which ever type that may be.
TYPES OF HARD DRIVE INTERFACES
There are only 2 types of interfaces for hard drives they are ST-506/412 or ESDI, other so called interfaces are SCSI or IDE, but both of these interfaces are system level interfaces that actually incorporate one or the other type of interface internally. ST-506/412 interface is an interface developed by Seagate Technologies and was designed for a specifiec drive the ST-506. This drive was a 6 megabyte full height drive in a 5.25 inch size and was developed in 1980. In 1981, Seagate made the ST-412 drive which added a buffered seek to the interface. This drive was 10mb and also 5.25 in full height. Also this drive was one of the drives origionally used in the IBM XT. Most of the major hard drive manufactures adopted this standard. Lastly this interface is not used with drives larger than 150 mb and so is not manufactured today.
ESDI or Enhanced Small Device Interface was established in 1983 by Maxtor Corportation and it was proposed as a new high performance standard. The drives preformance was increased by building an encoder/decoder or ENDEC into the drive. Also the drive was designed to be Plug and Play by giving it enhanced commands that allowed the controller to read the drives parameter's directly from the hard disk so that no system setup or parameters needed to be inputed.
IDE or Integrated Drive Electronics is a new name for an interface that has been available for some time, a Hard Card. An Ide drive has the controller built into the hard drives logic board so that it can be plugged directly into a port on the motherboard. The output connector signals are identical to the signals that would go to the Industry Standard Architecture Bus on the Motherboard.
SCSI or Small Computer System Interface is not a disk interface but a systems-level interface. It is not a controller but can be a number of controllers or a number of drives or devices of different types, it is more like a BUS then a hard drive interface. A total of 8 ports can be placed on the SCSI bus. Usually when you purchase ha SCSI hard drive you purchase the hard drive with the controller built in and a SCSI interface board which creates and controls the SCSI bus. The SCSI bus was standardized but not Dos SCSI Drivers. So not all IBM type SCSI hard drives work in the same fashion. Apple Computer did standardize the Scsi bus in their systems and standardized the drivers by embedding it into their ROM so all manufacturers could standardize Apple Scsi devices.
Besides hard drive interfaces there are also different types of DATA ENCODING SCHEMES used by hard drives. Encoding schemes are used in telecommunications for converting data into tones. In a hard drive the flux reversals are converted into digital data and back by an encoder/decoder called an ENDEC. The type of method used to encode/decode the data is the data encoding scheme. MFM and RLL are the types of encoding schemes used. MFM or Modified Frequency Modulation is a scheme in which a set pattern of bits always uses the same amount of space on a disk or it puts clocking information to the data that allows single bit errors to be detected. RLL or Run Length Limited is an encoding scheme where there is a larger interval between flux changes than MFM and no clocking information exists. RLL makes the controller have very acurate error detection since there is not clock timing information to detect errors. RLL allows for a higher data density so more information can be stored on a drive. Most newer drives IDE, SCSI, and ESDI use RLL encoding.