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Hard disk drives (HDDs) rely on a small read-write-head which hover about 70 nm over the platter 1, 2. The physical effect on which this hovering is based seems to be the ground effect 3, so it seems as if there is no electronic closed-loop control involved for vertical positioning.

Nonetheless, it seems that HDDs are actually quite resistant to physical shocks. I probably accidentally kicked my computer below my desk already a hundred times with running HDD and nothing bad happened. Also, HDDs for laptops would be practically impossible if the mechanical robustness was extremely low. But how does this work? Physical shocks in the nm range are probably quite often, and are hard to observe without special measurement equipment. Actually, I would expect head failures much more often than they actually occur.

Is the "air cushion" on which the head is hovering already good enough for achieving such great robustness or are there special tricks involved?

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    \$\begingroup\$ seems like not an electrical question \$\endgroup\$
    – user253751
    Mar 30, 2023 at 11:15
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    \$\begingroup\$ The forces that keep the heads flying are reasonable and the head and media can tolerate contact. Shocks can cause positioning errors as can noise from fire suppression systems and other sources. Yes, very robust compared toHDDs of old, but poor compared to a SSD. \$\endgroup\$
    – Kartman
    Mar 30, 2023 at 11:32
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    \$\begingroup\$ Most hard drive developers are probably electrical enginners so I guess this forum is a good place for it. Furthermore, I knew one electrical engineer who worked on similar stuff but unfortunately I lost contact. \$\endgroup\$
    – cakelover
    Mar 30, 2023 at 11:34
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    \$\begingroup\$ 50 years of constant development and 1+ billion mechanical hard drives made also adds up to big refinements. \$\endgroup\$
    – winny
    Mar 30, 2023 at 11:43
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    \$\begingroup\$ Note that the force acting on an object is proportional to the mass. The mass of the head is very small so even with large accelerations, the force is within the manageable range. \$\endgroup\$
    – Barry
    Mar 30, 2023 at 12:56

1 Answer 1

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The head positioning system is a voice-coil actuator. This is simply a coil of wire between two very strong magnets, identical in operation to a speaker's voice coil. Where a speaker is intended to faithfully reproduce acoustical movements up thousands of times per second, a hard drive's voice-coil accurately positions the heads anywhere within the defined range thousands of times per second.

It achieves ultra-high accuracy by continuously re-calibrating itself. This re-calibration eliminates drift from temperature changes, as temperature causes microscopic expansion and contraction of everything inside the drive, otherwise leading to alignment issues. Older drives which used stepper motors for the head position had difficulty adapting to temperature changes. So a modern drive is constantly readjusting itself for best performance.

The heads are "docked" or "parked" off the platter, using mechanical guides to lift them away from the platter. The heads are spring-loaded to exert some (small) amount of force towards the platter. Depending on the drive, it may spend most of its life in "parked" mode, where a kick wouldn't do it any harm. When accessed again, they move out of the parked area, complete the operation, then park again. It may sense a misalignment and recalibrate immediately, or wait for the next scheduled recalibration, which happens in milliseconds with one tiny blink of the indicator LED (if it even reports this as "activity.") Some drives may use a "parked" area and an "idle" area, which is on the disk surface but not used for storage. That would obviously be faster so may be more common.

Even if a kick happened during a read-write cycle, the "cushion of air" is already pushing against the spring pre-load tension of the head, so a fairly strong kick is needed to cause a "head crash." That is where the head "dives" into the coating on the platter and physically scratches it off. That, for one tiny spot somewhere random on the disc would mean a slew of "bad blocks" congruent to the scratch length and width. Barring any other damage, such areas would be marked "bad" by a (long, "surface") disk format and the rest would work normally. But if this happened on track 0, master boot record, partition table, or other special areas, the disk could be totally unusable.

The voice-coil can be seen briefly in this video.

P.S. In the past, it used to be that the physical components of a hard disk were more advanced than the electronics. Thus, a hard drive failure was usually a physical defect, and data was only recoverable in a cleanroom environment by swapping out the failed component(s) with good ones. But today, the control electronics are far more advanced, and fail about as often (if not moreso) than the physical components. Er.go. if a spinning-platter hard disk doesn't work today, there is a 50/50 chance that replacing the controller board (which is fairly easy to do and doesn't require a cleanroom) will get it running again. Of course, a controller failure could have thrashed and abused the physical components, so any such working repair should immediately recover the data and assume the drive may die again at any time. And, cooling the disk also sometimes works, by microscopically shrinking everything inside. But this should be considered a destructive process, because room moisture condenses on cool objects and that's true inside the disk as well - get the data before it dies.

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    \$\begingroup\$ Re, "head crash..." Last time I knew anything about how HDDs worked, the platters were coated with a thin film of diamond-like carbon. Its primary purpose, as I understood it, was to protect the magnetic film beneath from oxidation, but ISTR that it also helped somewhat to reduce the extent of damage from "light" head/platter contacts. \$\endgroup\$ Mar 30, 2023 at 13:44
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    \$\begingroup\$ It's not "recalibration" but rather voice coil hard drives are fundamentally closed-loop devices. Servo tracks are written at the factory on either a dedicated surface (old drives) or embedded alongside the data. Feedback control is used to continuously track the head position. \$\endgroup\$
    – user71659
    Mar 30, 2023 at 22:07
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    \$\begingroup\$ In addition to this, some modern hard drives-particularly those designed for mobile applications-include accelerometers. If they sense abrupt motion in any direction, the drive will stop what it's doing and lock down the read head at a safe height so that it's less likely to crash into the platter. \$\endgroup\$
    – bta
    Mar 30, 2023 at 22:15
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    \$\begingroup\$ I've overvolted HDD's in the past, and replacing the board didn't help. So I wonder if overvolting can also destroy the voice-coil actuator inside. \$\endgroup\$ Mar 31, 2023 at 10:45
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    \$\begingroup\$ @Crazymoomin possible. there are about three electrical parts that are not part of the board: the heads, the voice coil(s) and the motor \$\endgroup\$
    – user253751
    Mar 31, 2023 at 13:05

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