Question: On many hard drives, there’s a text warning to “not cover this hole”, sometimes adding that doing so will void the warranty.What is the purpose of this hole and why would covering it cause damage or increase the likelihood of drive failure?

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Answer: It allows for equalization of air pressure between the inside and outside of the drive. While it is not a complete pass-through of outside air into the HDD internals, there is a diaphragm filter inside the hole that allows the air pressure to equalize.

If the drive were completely sealed, operating at altitudes significantly different from those the drive was manufactured and sealed at would cause problems and increase the likelihood of catastrophic failures.

This system works in much the same way as the eustachian tubes that allow our ears internal pressures to equalize, preventing the explosion or implosion of our ear drums.

UPDATE: Per Moab’s correction, it’s a filter, not a diaphragm. The way it works and the reason it is included remains the same.