comme dit c_planet en version courte, voici la version longue
"Hard disks aren't sealed, because they have to be able to pass air between the inside of the drive and the outside, in order to equalize any air pressure differential that may exist between the two environments. This allows the disk to maintain proper equilibrium when the weather changes, or the drive is moved to a different altitude; if pressure is not balanced the drive might not perform properly and damage could even result. You can actually see the small breather holes in the cases of many drives, placed there for this purpose. Of course just putting a hole in the case would cause contamination of the drive, so the holes are covered with a breather filter which lets air pass through slowly but not dirt or dust. These filters are placed permanently and do not need to be serviced or replaced."