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HD Interface and Standards FAQ
All of todays many different types of hard drive (HD) have the same basic original components. The Head Disk Assembly (HDA) contains the platters, spindle motor, heads, and head actuator mechanism in one sealed unit. The Platters (upon which the data is actually stored) are usually made of aluminum or glass and generally come in two sizes 5 1/4 or 3 1/2 inches in diameter. Also it is not uncommon to have up to 15 platters in a single hard drive. The platters are coated with different types of magnetic substances which are applied using different methods. Oxide media based on iron oxide particles has been used since the 1950s. Though it is not used anymore on high capacity and high speed drives. Thin Film Media (TFM) uses a thin film of magnetic material (thinner than oxide media) and is used in today's higher capacity drives. TFM is also known as plated or sputtered media after the respective processes used to plate the media. Plated media is created with an electroplating process to coat the platter. Sputtered disks uses a process in which aluminum platters are coated first with a layer of nickel phosphorus and then with a cobalt alloy magnetic material within a continuous vacuum deposition process called sputtering. These two types of TFM media tend to be harder and can withstand a head crash (head to platter contact) better 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 30 heads in one drive. Each head is attached to a spring loaded arm that puts the head onto a platter (each platter has the heads above and below squeezed onto it). As the drive spins, the heads rise off of the platter(s) as a result of the air pressure developed under the heads. The gap between the head and platter at full speed is usually 5 to 20 millionths of an inch, which is why all head disk assemblies are sealed and not meant to be opened. A piece of dust inside the HDA could be hit by one of the read/write heads and result int he head "crashing" into the platter. Hard drives are built in clean rooms where a cubic foot of air cannot contain more than .5 micron particles (Class 100 Clean Rooms). Types of HD heads include: Ferrite heads, Thin Film heads, and Magneto-resistive heads. The largest and most common of these are Ferrite heads which have an iron oxide core wrapped with coils. An improvement upon Ferrite heads are, Composite ferrite heads that have a smaller ferrite core bonded with glass in a ceramic housing. Thin Film Heads are made using a method of sputtering iron and nickel on hard aluminum. Thin Film Heads are designed for a very narrow head gap and are four times more magnetic than Ferrite heads. Magneto-resistive Heads originally created by IBM are actually two heads developed into one. They contain a standard thin film head for writing and a special magneto-resistive head for reading. These MR heads can be three to four times more powerful than a thin film head during a read.
Along with the heads there is a Head Actuator Mechanism, responsible for moving the heads across the platter and positioning them over the needed data "block". The two basic types of head actuators include: stepper motor and voice coil. The Stepper motor type of head actuator is generally slow and alot less reliable. It also 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. By today's standards stepper motor drives are completely inferior to drives which use voice coil actuators. Most older hard drives which required "head parking" were based on stepper motor actuators. A stepper motor operates by moving from position to position, stepping with mechanical detents. Voice coil actuators operates using electromagnetic 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. In order to keep track of the position of the heads one side of one platter is designated as the track positioning platter. The data on (one side) of this platter is prerecorded to indicate the proper track. The head above the tracking platter contains only a read head (so that tracking information can not be altered). Voice coil actuators are accurate and fast, and also the auto park the hard drive.
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Thorin