How hard drives work

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How hard drives work

             We all use them to store our data on our PC or laptop.  These are amazing devices when you get into the actual functions they execute to read, write and recover our data for us.

             Hard drives store data in binary form, zeros and ones.  The actuator arm moves the head that is used as an electro magnet to write to the platter by changing the magnetization on specific sections of the platter and then reading what was put onto the platter by measuring the magnetic polarization; it requires a great deal of precision.  A "voice coil actuator" is used and the end of the arm sits between two powerful magnets.

The arm moves because of what is known as the “Lorentz Force”.  When you pass a current in a wire that is in a magnetic field, it experiences a force.  When you reverse the current, the force is reversed.  The position of the arm is fine-tuned by the current running through the coil.

             The head measures the charges in the direction of the magnetic poles.  As in Faraday's Law, when the head passes, a change in magnetization - either positive or negative - is recognized as a spike and represents a "one", when no spike is detected it represents a "zero".

In old hard drives, the head would float at 100 nanometers (0.00000393 inches) from the surface.  Today they float at 10 nanometers (0.000000393 inches).  The closer they get, the smaller the magnetic field which allows for more sectors of information to be packed onto the disk surface thus increasing capacity.

             To keep this critical height, the head floats over the disk.  As the disk spins, it forms a layer of air that gets dragged past the fixed head at 129 kms (80 miles) an hour in the outer edge.  To be able to float this close to the platter, the platter must be extremely smooth.  Surface roughness is about one nanometer (0.000000039 of an inch).

             The magnetic layer is made of cobalt with a bit of platinum and nickel added.  This mix has high coercivity which means it will keep its magnetization and maintain your data until you change it.

             Lastly, to be able to fit 40% more data, engineers have thought up a method called "Partial Response Maximum Likelihood" or PRLM for converting the weak analog signal from the head of a magnetic disk drive into a digital signal. To increase capacity, the magnetic domain is shrunk.  The trouble with this technique is that spikes sometimes overlap giving an imprecise signal.  PRML takes these imprecise signals and generates possible sequences then chooses the most probable.

             Hard drives need to be clean and remain clean.  That's why hard drives are sealed and air intake is filtered.  Particles entering the platter vault could damage the surface and cause data loss.

             Until a problem arises, the hard drive performs un-noticed.  But without its hard drive, your computer could not store all of the data you accumulate.

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