Hard drives have a lifespan of perhaps four to seven years, but there are a lot of factors determining their useful life and expected failure.
Some of these factors include...
Here is a small collection of videos and still photos depicting some things of interest pertaining to their inner workings.
Here is one picture of a hard drive running quietly, and another of a drive busy reading or writing (blur of motion).
The platters, not quite the size of a CD, are usually rotated by the motor and spindle at 7,200 RPM, though the range of other common speeds is 4,500 to 15,000 RPM. The actuator arm is pushed into precise motion over the platter surface by electromagnetism so the reading head (at the end of the actuator arm) can be correctly positioned over the platter to permit the writing and reading of data.
Both sides of a platter may be used for storing data. There may also be a stack of platters in a given hard drive. A matching stack of actuator arms and read/writing heads would be interlaced with the platters (gapped apart by perhaps an 1/8"), with electronics controlling the read/write sequences.
Note the apparent flawless mirror finish of the hard drive platters - you can hardly detect any motion whatsoever unless the heads are moving. The gap between the reading/writing heads and the platters is so small that particles of smoke and other impurities can be too big to fit in between. These particles, if they somehow get inside of the dust-free, air-filtered encasement, can be dragged around like the grains of garnet on some sandpaper and cause sufficient microscopic damage to the platters as to cause the drive to fail. The gap is small enough, and the mass of the actuator arm and read/write heads is great enough, that shock to the drive when the heads are not "parked" (away from the platters) may cause pitting and scarring of the platter's surface. (Think of it like dropping the needle of a turntable onto a vinyl record. Both the needle and the record may suffer damage.)
This link to a 29 MB video shows a hard drive running successfully. The drive is very quiet, and the motions are somewhat random. The hard drive is in the process of copying files.
This link to a 19 MB video shows a failing hard drive. You can hear the sound of a drive failing for the most part, but with occasional successful reads.
An example... If a drive fails to read correctly nine times out of every ten tries, but continues to read ultimately at this degraded rate - it will be performing at just 10% of its designed ability. With such a drive, the computer may continue to run and just seem incredibly slow.
This link to a 28 MB video shows a fully failed hard drive. You can hear the sound of the drive trying to read data and repeatedly failing.
This link is to a much smaller MPG video showing a hard drive failing. It might be enough for you to figure out what the internal sounds should, and shouldn't, be. As a general rule, the chunkier the noise, the more difficulty the drive is having. This drive is a 2.5" drive from a laptop - not much difference from the 3.5" drives in the other videos.
One way to effectively destroy the data on a hard drive you intend to dispose of is by dismantling the drive and removing the platters, and bending the platters into unusable shape.
A few things about disassembly...
1) drives may require the use of Phillips screwdrivers, but almost always the use of Torx-style screwdrivers, which are not always available in a household. Also, the size of the Torx screws are typically #6 and #8, of which #6 may be finer than Torx screwdrivers you may have around.
2) screws may be hidden under stickers and labels on the exteror of the drive casing. Removable tape may encircle the drive sealing the seam where top meets bottom.
3) the magnets inside a hard drive are very powerful - some parts are held together by these magnets, making their removal necessary. Do not pinch your fingers when peeling and prying these magnets off. Non-ferrous parts that may be inside will not stick the the magnets, regardless of their strength, of course.
4) disengaging and/or removing the actuator arm is generally required before removal of the plattters is possible.
5) the spindle generally has as its base the very precise electric motor that drives it - the motor is sometimes removable, but may be moulded, or riveted, in.
6) once the platters are removed from the spindle, you can recycle the excellent metals and the plastic, and dispose of the minimal electronics.
Read carefully, now. You are now ready for the destructive part.
To damage the platters beyond all but forensic access, here's what you can do...
1) round up a disposable bag of some size, a heavy pair of leather gloves, two pairs of heavy pliers, and safety glasses.
2) take a platter up with one of the pliers and insert your gloved hand, pliers and platter deep into the bag.
3) reach in with the other gloved hand and pliers and grab onto the platter from the opposite side.
4) look away while you lightly attempt to bend the platter, to see if it is the bendale type.
Big note: not all platters are bendable. Some of them break into fine flying shards like broken glass - see the picture below.
5) if the platter is bendable, it will take immense force to actually bend it into unusable shape. Bend and twist until you tire.
6) if the platter shatters, look carefully around for any shards that may have flown out of the bag. Carefully brush off your gloves and extract your hands and pliers. Close up the bag with all the broken parts found and contained, and dispose of it in a second bag.
Note: I have yet to see a bendable platter ultimately shatter, so if it starts to bend, expect it to continue to bend. The shattering kind take very little force to break, so you will know pretty much right off which kind you have.
Here is a picture of ruined 3.5" platters, followed by a picture of the larger portions of a shattered 2.5" (laptop hard drive) platter.
Other data disposal techniques include triple-overwriting over the entire drive surface with random data (at home, with special software), or degaussing (destroying the magnetic properties, done professionally), or grinding the drive up into bits the size of very coarse pepper (done professionally).
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