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"hard drive sealing system" - I'm pretty sure you can't completely seal a disk, it needs some surrounding air to deal with pressure changes and such.

Wonder if they can control the pressure to the point you can seal it... or if the seal itself incorporates an air flow system.

How complex can this sealing system be while still allowing the disks to fit in the bays?

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Pure speculation: The hole is for, as you said, pressure differences. The head uses air pressure to float above the platter. Changes in pressure could bring the head closer/farther away from the platter which could have unwanted consequences. Pressure fluctuations are due in part to changes in temperature. Since the temperature is regulated by the oil, and that temperature will change more slowly than with air, plugging the hole is relatively safe.

I'm all but certain disks are sealed. If air has a path to the platters then so does dust.

Air molecules are not dust-sized.

You can use a bladder to quickly equalize air pressure without exchanging air. (Air will slowly penetrate most bladders, but it's not fast enough for rapid pressure changes.)

Some heaters use a similar approach to limit corrosion. http://www.westank.com/bladder-tanks.php

That's what I was thinking too, and the pressure changes shouldn't be so drastic that the bladder would burst.

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