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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?

Do not cover this hole (Image source)

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    I especially appreciate that you took a picture of a MacBook Pro where the ribbon cable to the front infrared board and sleep sensor generally covers "THIS HOLE" but presumably not in an air tight manner.
    – bmike
    Dec 16, 2011 at 23:14
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    By the way, if you ever build one of those silent oil-submerged computers, DO COVER THIS HOLE!
    – wim
    Dec 19, 2011 at 2:27
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    You'll need a snorkel to extend the hole to an uncovered position.
    – XTL
    Mar 7, 2012 at 12:47
  • Update in 2018... better to use m.2 SSDs or SATA SSDs in oil-submerged computers nowadays. Apr 24, 2018 at 18:17

4 Answers 4

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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.

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    Its a filter not a diaphragm, at least on the few hundred I have disassembled. Air actually does move in and out of the hard drive when temps vary.
    – Moab
    Dec 16, 2011 at 21:07
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    Here is an excellent teardown of a HDD and a description of how it works (including the filter.) Dec 16, 2011 at 22:09
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    @surfasb I harvest the super magnets out of them, lots of fun for older kids to learn about magnetism, If I was a real nerd I would make some sort of invention out of them. Had a few older drives the platters were made of glass like material, man did they shatter.
    – Moab
    Dec 17, 2011 at 0:40
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    @jfgagne I'm not sure your numbers are going the right direction: If pressure decreases, any given volume of air would increase as the altitude rises. Using your 20% number, that would mean that 20cc of air at sea level would be 24cc at "airplane elevation". This is further illustrated by snack bags, which expand at altitude. Dec 27, 2011 at 14:21
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    They do make custom HDDs filled with helium which are, by necessity, sealed. Helium has 1/7 the density (thus the drag) of the air which allows thinner plates and air gaps (allowing to fit more plates into a standard-size enclosure) and lower power consumption for spinning the plate stack and positioning the heads. The downside, apart from the stricter external pressure limitations, is the small size of helium molecules that are very hard to keep from sipping outside. Mar 23, 2017 at 19:49
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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.

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    What happen if the hole is accidentally covered?
    – deathlock
    Mar 17, 2013 at 11:14
  • If the harddisk requires a certain air pressure to operate correctly, then why isn't it sealed airtight at the right pressure? Then it would work everywhere (eg: even in a vacuum or high pressure environment).
    – compie
    May 15, 2014 at 14:28
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    @compie the pressure difference would deform the enclosure which for such a high-precision device would mean certain malfunction. Instead, for cost efficiency, they make rugged reinforced cases for such applications. Mar 23, 2017 at 19:43
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It allows for equalization of air pressure between the inside and outside of the drive. In other words, it keeps the air pressure at the same pressure as atmospheric pressure.

A hard drive is only designed for a certain range of operating pressures. The read/write head floats above the platter on a cushion of air. 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.

If the air pressure should be kept constant, why make a hole that allows the air pressure to change?

The hard drive can be used in a range of environments with different temperatures, including ambient temperature and operating temperature. If the drive was completely sealed, the temperature differences would cause large pressure variations in the hard drive. The variation in atmospheric pressure is relatively small compared to these differences.

Also, if the hole was blocked, the pressures could cause the case to bend and the spindle and arm to go out of alignment (theoretically; hard drives look quite solid).

There is another consideration: The hard drive may not be completely air tight apart from the hole, although I'm not sure whether actual hard drives are built this way. In this situation, the hole acts as an easier pathway for air to flow so that the air flows through the filter rather than through unfiltered cracks that allow dust to enter the hard drive.

The above being said, sealed hard drives exist, which have mechanisms to deal with the pressure changes.

Note about the discussion in the other answer: If the drive were completely sealed, operating at altitudes significantly different from those the drive was manufactured and sealed at would have no effect at all (at the same temperature) because the hard drive is a fixed volume so the internal air pressure is unchanged.

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    Part of this answer appears to be an unsourced direct quote from wikipedia (assuming the other answer that does attribute it is correct). Please properly acknowledge your sources.
    – Ben Voigt
    Jun 3, 2015 at 15:28
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The breather hole allows condensation inside the hard drive to escape and it also equalizes the hard drive's internal pressure with the ambient pressure. The hard drive needs it to function properly, so make sure you won't occlude the hole.

Source: Samsung EcoGreen Hard Disk Drive Review at Techarp

On Wikipedia we can read:

The HDD's spindle system relies on air density inside the disk enclosure to support the heads at their proper flying height while the disk rotates. HDDs require a certain range of air densities in order to operate properly. The connection to the external environment and density 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 density 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.

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 air inside the operating drive is constantly moving too, being swept in motion by friction with the spinning platters.

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