In the event there is no power, heads touch magnetic surface because of spring. After disks spin up, increasing aerodynamic pressure takes heads off from the surface.
Due to a tiny gap between writing surface and head, head disk assembly can be disassembled only in clean rooms. Storage media assembly is done in clean rooms of Class 100. This value indicates that a cubic foot of ambient air can contain less than 100 fine dust particles (up to 0,5 micron).
Maintaining such sterility requires a lot of money. Some companies offer “clean boxes” that can be put on top of a table. They are relatively cheap and look like big boxes with plexiglas transparent walls fitted with special gloves. Before starting the work an operator should put the required tools and devices inside the box and after that air-filtering system should be activated. After some time the purity of inside atmosphere will reach the desired indicators and only then you can start the repair.