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Hard Drives: How They Work (Page 3/4)

Posted: April 17, 2000
Written by: Tuan "Solace" Nguyen

Read/Write Head:

The actuator in motion:

The read/write head is attached to the ends of the actuator arms. From the above picture, it looks like the heads are touching the platters. But in actuality, they are “flying” above the platters using a cushion of air created by the spinning platters. This thin layer of air is what prevents the heads from coming in contact with the platters. So how close are the heads to the platters exactly? Try 2 microns. Below is a diagram of what 2 microns is like:

As you can see, the heads are extremely close to the platters. An excessive shock to the drive during operation can mean disastrous outcomes. Also, remember how we mentioned that these components are sealed in a dust-free chassis? If the heads are flying only about 2 microns above the surface of the platter, can you imagine the consequences if any small particle were to come between the head and the platter?

The heads are responsible for writing to the drive and reading from it. They write data by arranging the magnetic particles on the surface of the platters. When arranged in one direction, the particles will produce a northern polarity and when arranged in the other direction, they will produce a southern polarity.

When reading from the platters, the head will detect the polarity of the particles and translate that into electrical signals and send the signals back to the on-board hard drive controller.

The hard drive controller is attached on the exterior of the hard drive chassis. It contains DSP (Digital Signal Processors) chips that control dataflow, etc. It also contains cache modules to keep frequently used data close at hand.

The mini PCB inside the hard drive itself is the controller for the actuator.


You may think that placing a magnet near a hard drive will destroy the data inside. This is true. But what you may not know is that there are two very strong magnets inside of the drive. The magnets are located inside the actuator. One is on top of the actuator arms and the other one is underneath. The reason why these magnets do not affect the platters is that the magnetic field on the magnets lie on a vertical axis. If they were on a horizontal axis, the field would stretch onto the platters, destroying the data. Another reason is that the magnetic signals recorded on the platters aren’t any regular magnetic fields but they are electro-magnetically recorded. Electro-magnets are many times more stronger than natural magnets.

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