All hard drives share the same basic structure, varying only in how each part is used and the quality of the parts themselves. The platters, spindle motor, heads, and head actuator are inside the drive, all sealed from the outside environment. This chamber is often called the head disk assembly (HDA). The HDA is rarely opened, except by professionals. On the outside are the logic board, bezel, and mounting equipment. Below, I will describe each of these components.
The platters are the disks inside the drive. Platters can vary in size. Often the size of the drive, 5.25″ or 3.5″, is based on the physical size of the platters. Most drives several platters. They are usually made of an aluminum alloy so that they are light. The newest and largest drives make use of a new technology of glass/ceramic platters. Basically, this is glass with enough ceramic within to resist cracking. This glass technology is taking over aluminum in the hard drive industry. Many popular manufacturers already use it, including Maxtor, Toshiba, and SeaGate. Glass platters can be made much thinner than aluminum ones, they can better resist the heat produced during operation and they are also better able to withstand the extreme centrifugal forces during spinning on the spindle. The platters are mounted onto a spindle in the interior of the HDA.
Alone, platters are not capable of recording data. Each one is coated with a film of some magnetically sensitive substance. The oxide media is one of the older ways of doing this. With this, a mixture of compound syrup is poured onto the platter, then spun to evenly distribute the film over the entire platter. This substance has iron oxide as a main ingredient, explaining why many platters you may see will be brownish-orange. Using an oxide coating developed limits over time as capacities increased. Whats more, the oxide medium could not survive a head crash at all, and usually this required drive replacement. The more modernly used media consists of a thin film of a cobalt alloy which is placed on the platter through electroplating, much like chrome. This media is then coated with a thin layer of protective substance to allow some measure of protection against head crash. Overall, this new medium is much flatter at the microscopic level, allowing the heads to run closer to the platter (more on this in a bit).
The read/write heads do just that, they read and write to the platters. There is usually one head per platter side, and each head is attached to a single actuator shaft so that all the heads move in unison. Each head is spring loaded to force it into the platter it reads. When off, each head rests on the platter surface. When the drive is running, the spinning of the platters causes air pressure that lifts the heads ever-so-slightly off the platter surface. The distance between the head and platter is very small…so small that the HDA must be assembled in a clean room because one dust particle can throw the whole thing off. This sensitivity and accuracy is what causes only bigger companies to be able to repair hard drives simply because of the expense of a clean room. A slider is attached to each head. This mechanism actually glides over the platter and holds the head at the correct distance to do its job. You can see a full head assembly to the right. You can see the sliders on the end of each head.
Air flow inside of the drive plays an important part in overall operation. In fact, air is what holds the heads off of the platters while the drive is on. The rotation of the platters creates an air flow. The heads fly like an airplane on this cusion of air. The air is dragged along with the platters by friction. The high pressure air between the heads and platters form what is called an air bearing. The concept is similar to a hockey puck gliding over an air hockey table.