A harddisk essentially consists of a flat plate like object that is polished to achieve a mirror- like effect. This is the hard disk’s platter, and this is where the data is actually stored using the magnetic properties of the materials the platter made of.
Hard disks of today feature multiple platters. Each platter usually has read/write head attached to it, which in turn is attached to an arm or actuator that moves the head over the platter.
Writing is achieved by subjecting the platter to a magnetic flux by the write head, which lead to the change of polarization on the platter. The process is reversed for reading – the magnetic field of the platter affects that of the read head, and this is interpreted as data.
A platter and a head can be thought of as the gramophones and record players of yesteryear, where the needle moved on the record to read the record and play the song. The case is pretty much the same here, except the read/write heads donot touch the platter- they fly less than a hairbreadth above it.
The platter can spin at roughly 3,000 inches a second, which is approximately 270 kmph! Today, hard disks generally run at 5,400 or 7,200 rpm, though you do get hard drives where the platter spins at 15,000 rpm.

 

 

 

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OK I would like to add few things to your writing. Hard drive speed of today is set at 7200 rotations per minute rpm. And 5400 rpm is considered to be too slow also if you consider the fast ram (RAM - random access memory) h owever there are are also faster hard drives if you need intensive writing for your programs usually these are used on the storage farms and so on. Or if you hvae some computer acting as server then you would use faster drive there because you want your sites to be responseve as they can.

Today the slowest component in the computer is hard drive and this is very interesting because it is developing relly slowly and this is the main reasons why it is the bottle neck of todays computers architecture.

Also I would like to point out very important thing and that is new breaktrough in the way how data is written to the hard drive plate. Commonly used way was to write horizontally however if you have hard drive that has more then 500 GB then it most certainly uses vertically way of writing to it. Vertical way makes it possible to write more data and searching for the data cylinderes is much faster then in the usual horizontal way. Also Because of the more dense structure more data may be stored allowing disks up to 1 TB on a single plate...

 

 

 

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