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About Drives

Hard Drives- How They Work

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. Each of these components are described below:

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 have 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, and 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. What's 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.

Airflow 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 airflow. The heads fly like an airplane on this cushion 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.

The read/write heads are bonded to what are called head arms. The head arms hold the heads over or under the platters. The head arm and head combination is called the head-gimbal assembly (HGA). The individual HGA's are mounted together onto an actuator shaft, which serves as a center of rotation. The head actuator is situated on the other end of the HGA and is responsible for the movement of the heads around the platters.

Head actuators come in three types:

  • The stepper motor design is actually an electric motor that moves from one stop position to another, governed by click stop positions. They cannot stop between stop positions. The motor is small and is located outside the HDA, so it is visible from the outside. The stepper motor design is inferior. It suffers from slow access rate and is very sensitive to temperature. It is also sensitive to physical orientation and can't automatically park the heads in a safe zone. Besides, the actuator operates blindly from the track positions, governed only by the stop positions. Over time, the drive becomes misaligned, requiring occasional re-formats to realign the sector data with the heads.
  • The servo motor actuator is another type of head actuator motor. Unlike the stepper design, the heads get feedback as to position, assuring the proper tracks are read. The guidance system used by the heads is called a servo. Its job is to position the head over the correct cylinder. It does this through the use of Grey code. Grey code is a special binary number system in which any two adjacent numbers provide info to the servo as to their position on the drive. Also, the heads are free to move wherever needed...no steps. Basically, when the drive needs to retrieve certain data, the servo motor moves the heads out to the appropriate position on the disk and then waits for the corrects bit of data to spin over to it. The time it takes for all this to happen is called latency, and is a key measure of the speed of the drive.
  • The voice coil is the latest type of device for controlling the heads and is used on all modern hard disks. The voice coil operates similarly to a speaker. There is a magnet (or magnets) that is surrounded by a spring-loaded coil of wire, which is connected to the HGA. As a current is applied to the coil, it interacts with the magnet and swings the assembly. The resulting movement of the heads is from the center of the platters to the outer edges.

When the hard drive is powered down, the springs or actuator coil (depending on the type of actuator) attached to the heads pull the head into the platter. This is called a landing. Every drive is designed to handle thousands of takeoffs and landings, but since the head actually hits the platter, its best to have this happen on a section of platter where there is no data. In a voice coil design, the actuator coil springs the heads into a landing zone and lock position before the drive even stops spinning. The landing zone typically lies on the innermost cylinder or the outermost cylinder. This assures that the heads are not just let go of and left to drag along the platter until the platter stops, a problem common to the stepper motor design. When powered on, the drive automatically unparks itself and the coil is overcome by the magnetic force.

The spindle motor is responsible for spinning the platters. These devices must be precisely controlled and quiet. They are set to spin the platters at a set rate, ranging from 5400 RPM to 10000 RPM. The motor is attached to a feedback loop to make sure it spins at exactly the speed it is supposed to. The speed is not adjustable during operation. Some spindle motors are on the bottom of the drive, below the HDA, while the more modern ones are built into the hub of rotation of the platters, thereby taking up no vertical space and allowing more platters. Attached to the spindle motor is a ground strap, which helps rid the drive of the static charges, created by the rotating the platters through the air in the drive's interior. In many drives, this can be accessed by removing the logic board. After a while, this strap can become worn and produce noise, like a high-pitched squeal. One can usually lubricate the strap and stop the noise, but this entails some minor disassembling of the drive.

The logic board is the board of chips underneath the drive. It controls the spindle and head actuator and also translates data to a form usable by the controller and the rest of the system. Some logic boards have an integrated controller, also. Sometimes, an apparent disk failure is actually a failure of the logic board. In such a case, you can replace the logic board and regain access to the data held up on the drive. This is relatively easy to do, because the board is simply plugged into the drive and held in by screws.

Data Storage
I have discussed the structure of a platter itself, but how exact is data stored on that? First, the heads are actually small electromagnets. To write information to the platter, a small electrical current is pulsed from the electronics of the drive to the head. The direction of the current, and thus the direction of the magnetic field between the head and the platter, determine whether or not the data bit is a 1 or a 0 in binary (which is the only language a PC understands). This magnetic field alters the direction of the little magnets on that portion of the platter. The "little magnets" are actually microscopic, the size of a molecule. The direction of the magnet is retained over time and remains that way unless rewritten to reflect something different. Thus, the hard drive will retain this data even after it is powered off.

Capacity
Just take a trip to the computer store and you can see that there have been major advancements in hard drive technology that lead to larger capacity drives. Where there were once 500MB drives, we now have huge drives. Recently, a 36GB IDE drive was released. What led to this?

Well, the first thought would be: Add more platters, or maybe bigger ones. Well, yeah, larger platters would do the trick. 5.25" platters have been used on older drives, and do hold more data. But, manufacturers don't use these big platters because of the extra stresses the larger platters put on the motor. These stresses, and the simple fact that the heads have more disk to cover, make the drives hotter and a lot bigger. Most drives in use today use 3.5" platters, and 2.5" or smaller is commonly used for notebook systems. So, the manufacturer decides it would be better to keep smaller platters, but just add more of them. This works, but in order to reach such high capacities, you'd need a lot of platters. The vertical heights of these drives would just be too much.

Keyword: Areal Density. This is the closeness of data bits on the hard disk to each other, usually measured in megabytes per square inch (MBSI). There are a few factors that go into determining the degree of areal density that is possible:

  • Size of the magnetic particles in the platter itself
  • Size of the read/write heads. Smaller heads allow larger densities.
  • Altitude" of the heads over the platter surface.

This density increases performance and allows more data to be packed onto a platter. The closeness of the bits together means that more data passes the head at a time, increasing read/write performance. By combining this with increase platter rotation speeds, latencies will be lower and speed will be higher.

Information source: PCMECH.COM


 
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