What Is the Purpose of the “Do Not Cover This Hole” Hole on Hard Drives?

From tiny laptop hard drives to beefier desktop models, traditional disk-based hard drives have a very bold warning on them: DO NOT COVER THIS HOLE. What exactly is the hole and what terrible fate would befall you if you covered it?

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-drive grouping of Q&A web sites.

The Question

SuperUser reader oKtosiTe noticed the warning label and needed to get to the bottom of things:

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?

Thankfully no field studies or warranty voiding were required to solve the mystery.

The Answer

SuperUser contributor music2myear offers some insight into the tiny hole and the importance of leaving it unobstructed:

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 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, it 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.

Dennis expands on this explanation by directing us towards the section of Wikipedia dealing with hard drive integrity:

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.

The mere mention of headcrashes (and the horrible memory of the sounds our last lost-to-head-crash drive made) are more than enough warning for us.
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