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Air Circulation and Air Filtration
As discussed in the section describing hard disk head flying height, the old myth about hard disk heads and platters being "in a sealed vacuum" or whatnot, is wrong in two ways: there's no vacuum in there, and the drive itself isn't sealed, at least not in the way most people believe. In fact, air is an essential component for proper drive operation. Regular hard disks aren't totally sealed from the outside air, but they definitely are separated from it, in order to ensure that the dirt and dust of the outside air is kept away from the delicate platters and heads of the drive. If foreign matter were to get onto the platters--even something as small as smoke particles--a head crash or other problems could easily result.
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.
Hard disks also have an internal air flow within their sealed chambers (caused by the rotation of the platters--there is no fan inside a hard disk). This air flow is used to continuously filter the air within the disk assembly. Despite building the hard disks in ultra-clean facilities and taking other precautions during manufacturing, a small recirculating filter is built into the drive itself as an added security measure. This filter is designed to work on the air that flows within the hard disk assembly, catching any minute bits of debris that might somehow make it inside. This reduces the chances that such dirt will end up on the disk platters. Like the breather filter, the recirculating filter is not changeable, nor does it need to be.
Next: Orientation and Mounting