This is a video that shows the working of a hard disk during various manipulations. You can see the start-up of the disk with the reading of the SA field, the the hard disk working during copy and files deletion. In the end, you can observe what we talk about as a hard disk crash with destruction of the surface and faulty read head.

It rarely happens that hard disk are so dirty that we have to clean them before any other manipulation. In order to do that, we use different kind of chemical products, of which composants are secrets. This leads to éliminate impurities that are on the hard disk, without changing the hard disk mechanics.