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The hard disk drive (HDD) is the part of your Personal Computer that stores data even when your computer is turned off. The first PCs offered no way to store data. Early personal computers required users to load the programs they wanted to use every time they turned on their PC. Advances came quickly since the lack of storage capacity made the early PCs user-unfriendly. At first there were cassettes that stored data on tapes. These were followed by the floppy disk introduced by Apple Computers.

Tape drives and floppy drives had a very limited storage capacity. Mainframes had huge amounts of storage available. Direct Access Storage Devices were very large and expensive and while it was OK for mainframes, personal computers needed something a little more convenient. IBM developed the first hard drives strong enough for permanent use in PCs. The IBM Winchester was the first to use a read-write head that avoided actual contact with the disk. This read-write head prevented wear and tear on the disk itself, a big improvement over earlier versions.

Many advancements have been made since the 1980s when disks of the Winchester type were popular. Hard drives have become a lot faster and allow a lot more storage. The way the disk interfaces with your PC has changed and the mechanics and electronics of hard drives are now much more optimized and automatic. Even the physical size of the fixed disk in your computer today is significantly smaller than early HDDs. They are a lot less expensive too.

All hard drives are built in basically the same way, differing only in quality and how each part is used. The platters, spindle motor, heads and head actuator are sealed inside the drive chamber. This head disk assembly (HDA) is rarely opened, except by qualified technicians. Outside of the HDA are the logic board, bezel, and mounting equipment.

THE PLATTERS are the disks inside the drive. Platters can vary in size and the size of the drive is dependent on the physical size of the platters. Most drives have two or more platters with the larger drives having more. They used to be made of a light aluminum alloy and lately of new glass/ceramic platters. Glass technology is becoming the norm because they are thinner than the aluminum and more heat resistant.

Each platter is coated with a film of magnetically sensitive material. A compound liquid is poured on the platter, then spun to evenly distribute the film over the whole platter. Iron oxide is often the main ingredient, which is why many platters may be brown/orange in color. The other media is a thin film of a cobalt alloy installed on the platter by electroplating.

THE READ/WRITE HEADS read and write to the platters. There is usually one head per platter side, and the heads are attached to one actuator shaft so that all the heads move together. The heads are spring loaded to force them over the platter it reads. When off, they rest on the platter surface. When the drive is running, the spinning of the platters causes the heads to rise slightly off of the surface. A slider is attached to each head to hold it at the proper distance to read and write.

THE HEAD ACTUATOR is what the heads are attached to and moves the heads around the platters. There are stepper motor actuators and servo motor actuators. The stepper motor is an electric motor that moves from one stop position to another indexed by click positions. The drawback is that they cannot stop between stop positions. The motor is located outside the HDA, and is visible from the outside. The stepper motor is very temperature sensitive. After a while, the drive becomes misaligned; requiring disk re-formats to realign the data with the heads.
The servo motor actuator is found in newer drives. The heads get feedback information for positioning, and the right tracks are read. The guidance system used by the heads is called a servo and its function is to position the head over the correct cylinder. It does this through the grey code. They grey code is a binary number system in which any two adjacent numbers help guide the servo to a position on the drive. The heads are free to move where needed without steps. When the drive needs to retrieve certain data, the servo motor moves the heads into position and then waits for the correct data to spin over to it. Latency is the time it takes for all this to happen and is the primary measure of the speed of the drive.

THE SPINDLE MOTOR spins the platters. These motors must be precise and quiet. They spin the platters at a set speed, ranging from 5400 RPM to 10000 RPM. The speed is not adjustable. Some spindle motors are on the bottom of the drive while the modern ones are built into the hub of the platters. A ground strap is attached to the spindle motor which helps get rid of the static charges created by the rotating platters. The logic board is the board of chips underneath the drive. It controls the spindle and head actuator. Some logic boards have an integrated controller.