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Hard Drive Platters

hard drive with scored platter surfaceThe platters of a hard drive actually hold your data. The image to the left illustrates what a platter looks like after it has been scored by a head crash.

The magnetic surface of each platter is divided into magnetic regions less than a micrometer in size. When talking about hard drives, it’s important to note that everything is measured on a microscopic level. Click here to read more about the complexity of modern hard drives. Each of these regions makes up a single binary unit of information. A binary region on a hard drive platter (as of 2006) is about 200-250 nanometers wide (within the radial direction of the platter) and extends about 25-30 nanometers in the down-track direction. This comes to about 100 billion bits (100 gigabits) per square inch of hard drive platter area. The materials which make up the magnetic layer are usually a cobalt-based alloy. Today, each of these magnetic regions within the hard drive is composed of a few hundred magnetic grains. It is these magnetic grains which become magnetized when data is written to the drive. It is possible that in the future, drive manufacturers may use different systems to create the magnetic regions.

What Are Hard Drive Platters Made Of?

The actual platter is most often made of aluminum or glass substrate. After the platter is machined, it is then coated using a vacuum deposition process called magnetron sputtering. Vacuum deposition is a wonder all on it’s own, as this process deposits material, atom by atom or molecule by molecule onto a solid surface. The coating itself is comprised of mostly non-magnetic metallic alloys which is very complex in structure. There is also a protective coating that is carbon based and is applied in the same manner. Finally, a thin lubricant layer, approximately one nanometer is applied by dipping the platter into a solution. The platter is then allowed to cure and receives a final buff to remove any defects in the surface. A special sensor examines the platter surface for any remaining defects.

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Maintaining Alignment Of Multiple Platters

If you’ve ever seen the inside of a hard drive, you may notice that in some drives there are multiple platters.  There can be as many as 4 in the more common drives, but that is typically the maximum.  On average, depending on capacity, you will find 1 to 3 platters in a hard drive.  Each surface will have a corresponding read-write head, and the data is spread out among each platter.  The amount of data each platter holds and the mapping of the heads varies from model to model.

Maintaining the alignment between the platters is crucial.  Data will probably be inaccessible, and may be unrecoverable, if the platter alignment is lost.  The vertical and horizontal rotational alignments must be maintained at all times and there are only nanometers of tolerance.  So if you or someone you know have disassembled your drive and removed the platters without maintaining their initial alignment, chances are your recovery just became considerably more difficult.  This video demonstrates how important platter alignment is…

Special Care When Recovering Data

Platter surfaces have a mirror finish. Any contamination can render the contaminated area of the platter unreadable. The surface of the platter is especially susceptible to finger prints with even the slightest touch. That is why careful handling of a hard drive is critical during the data recovery process, and all recovery work must be completed in a clean room environment.

When a hard drive suffers a physical failure, such as a head crash, the platters can sometimes become scored due to the head coming in contact with the platter and grinding the surface. In these cases it is rare that data will be recoverable.


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