Hard disks are not sealed. Disks from the 1960s were. Then engineers realized they could dramatically reduce the amount of structural bracing if they left a "breathing hole" open and allow air pressure to equalize. This is why all hard drives have that little 2mm dot labeled "Warning do not cover!".
Even more amusingly, hard drives need that air inside to operate and they won't work at high altitudes.
Even more amusingly, hard drives need that air inside to operate and they won't work at high altitudes.