Why is it that engineers have been able to put 80 gigabyte onto one platter when a 80 gigabyte hard drive was at one time the size of a small car? We need to understand a little about density.
Rotational Speed, the idea behind this is the linear speed of the drive. The faster the better. We want our signal strength to be the best as possible. When the head actuator moves the the inside of the drive it is also moving data slower. We want to compensate with the speed of the out portion of the disk. Again the faster the better. Most disk run at least 7200 Revolutions Per Minute (RPM). Insure you buy at least this RPM rating. There are also drives out there that can spin at 10,000 RPM and again faster is better. Take note that the faster it spins the more heat you will produce, get some fans.
Read/Write Heads, this technology seems to limit density for the most part. Even if you put large amounts of data on a platter this doesn't mean that the heads can read it. As you pack more data bits in a smaller place the signal strength deminishes of the bit itself. We need to make use of a better read/write head to do this. There are also ideas of making the read/write head closer to the platter and even letting it rest on the platter. If allowing the head to rest on the platter we will have more wear and tear and drive life will be shortened. So with that said we need to improve the signal strengths within the head itself. There is a technology which helps with this that is called Magneto Resistive (MR). This actually sends a constant current through the head and when passing data bits the current will fluctuate indicating signal strength of that data bit. This is simply a way to amplify a existing signal.
Disk Media, the platters need to be made of light weight materials that are also resistant to damage. Most platters are made up of ceramics, glass, aluminum and combinations of all of them. The idea as discussed earlier is to make them light and lower heat potential. The actual layer that holds the data is called a cobalt layer and is between 1 and 3 microinches. This is getting kinda thin now isn't it. As you can tell everything has to work close together.
After looking at the read/write heads and tracks, sectors, cylinders, rotation speed, and what the platters are made of we have to wander how it can survive. This is really just physics but man do they work well together.
Next: Drive Controllers
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