Before moving into how the HDD operates, I thought I would talk about the basic principles of magnetic recording.

Remember the good old cassettes & cassette player\recorders?

HDD is a digital equipment and the data to be stored used binary, ie., we write only 1s and 0s to the disk.
Now the question is, how do we represent 1 and 0 on the magnetic media?
Ans: We make use of the polarities of the magnetic media. We polarize a set of particles to point to one direction to represent 1, and partcles representing 0 to the opposite direction.

The figure below would explain it better:



This figure shows a writing sequence.

The Drive Channel converts the input from the computer to electric currents in the little head coil.
The current in the coil reverses at each 1, and remains in the same direction for a 0.
The interaction of the head with the media results in the magnetization of it in a direction depending on current direction.
The media is broken in to blocks and block size is defined.
Hence, if we write a 1 in say Direction A on block #1, if next input is 0, block #2 will also be magnetized to direction A. But if second input is 1, block #2 will be magnetized to direction B.

The reading process includes excitation of the current in the head coil when the head "senses" changes in the magnetic flux.
The read voltage pulses at the flux transitions are then translated into sequences of bits equal to 0 and 1.
The so-called Wallace's spacing loss factor postulates that the loss of magnetic signal power will be proportional to the media - head separation.
This requires magnetic heads to fly as close to the disk surface as possible, which forces modern heads to fly at a few nanometers only (compare this to 20,000 nanometers back in 1957!).

...to be contd.