Do not cover this hole?

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?

(Image source)

Answer 1

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 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.

Answer 2

Check out the Wikipedia hard drive entry paying attention to the Integrity section with reference to the "breather hole":

Hard disk drives require a certain range of air pressures in order to operate properly. The connection to the external environment and pressure occurs through a small hole in the enclosure (about 0.5 mm in breadth), usually with a filter on the inside (the breather filter). If the air pressure is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about 3,000 m (9,800 ft).[99] Modern disks include temperature sensors and adjust their operation to the operating environment. Breather holes can be seen on all disk drives—they usually have a sticker next to them, warning the user not to cover the holes.