I feel that my question is probably too wide, I'm willing to focus my question more if needed and possible. Please comment.

I have a hard-drive (2.5" Toshiba) which was installed in a notebook. The notebook fell from a table (on the side) while running. Since then the disk makes the click-of-death-sound and is no longer recognized either by the system or when attached to a SATA-USB-adapter on another PC.

In the past, for another hard-disk with the same symptoms, I did/tried a transfer of the hard-drive-heads from a working disk to the non-working one. I was unable to make the broken disk work again - the click-of-death remained. The data was lost.

At that time I only transplanted the hard-drive heads. I was as careful as possible. I saw that the heads of the broken drive were visibly damaged. The donor-drive was the same model, but not the same creation date.

When done, I let the disk run while being semi-open. I saw what created the click-of-death: it's the heads running over the whole disk and then returning abruptly onto their parking position - this is when it is clicking. The controller instructed the drive to do it again and again. It was surely looking for some "known-data".

Now, for my current broken disk, I have a working model of the same revision and date ready to be a donor. But I'm unsure - this time I really would love to get back the data.

What causes the controller to scan the disks again and again? What is it looking for?

Why does replacing a physically damaged head not help?

Is there some kind of calibration stored on the PCB which is adapted for the heads?

Transferring the heads was relatively easy. Was it too easy? Did I probably break something, even though I was really careful?

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    Transferring the heads? Are you doing this in a clean room? – DavidPostill May 21 '17 at 21:15
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    I seriously recommend going to a professional data recovery store. They might charge a pretty penny, but they'll get you're data back... – undo May 21 '17 at 21:20
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    @Rahul2001 Maybe. As I mentioned in my answer, the OP's attempts to "fix" the drive may have damaged it beyond any hope of recovery. – duskwuff -inactive- May 21 '17 at 21:21
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    Back in the day when a large hard disk had a capacity measured in Mb not Tb, somebody with steady hands could indeed swap a set of disk heads, and align them just by gently moving the heads around with the fixing screws slightly loose, until the "clicking" stopped and the drive controller had locked on to the "guide data" tracks that were permanently recorded on the disk. But that doesn't work any more, unless you are very lucky indeed. – alephzero May 22 '17 at 1:34
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    "Did I probably break something?" Most probably. On a modern disk, the clearance between the heads and the disk is less than the depth of a fingerprint. A "speck of dust" on the surface is the same sort of disaster as flying a 747 into the side of a mountain! In fact, some recent high capacity disks are hermetically sealed and filled with slightly pressurized helium, not air, to handle these tiny mechanical clearances more reliably. – alephzero May 22 '17 at 1:43

The "click of death", in hard drives, isn't a single specific problem. It's a symptom which is often displayed by a failed hard drive -- the "click" is the sound of the drive resetting the heads after it fails to read data from the disk. This can be for any number of reasons: it can happen if the heads or the amplifier circuitry on the arm are damaged, if the receiver on the logic board is not working properly, if the disk is missing calibration data, if the drive is not spinning at the right speed, if the drive is failing to read its firmware from the disk... it can really be the result of almost any fault.

Do not try to transplant heads between drives. This was sometimes possible in the past, but tolerances in modern hardware have shrunk to the point that it's no longer possible to remove and replace the heads by hand. You will damage the heads, and the surface of the disk, if you attempt to do so.

If you believe that the mechanical components of your hard drive have been damaged, and you want to recover the drive's contents, contact a professional drive recovery service. Anything you do will only make the damage worse, and may render it unrecoverable even by a professional.

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    With the right tools (like plastic spacers to lock the heads) it is possible to do this by hand, however without experience the first 21 attempts will fail, and of course something at least resembling a cleanroom will be necessary. There are a couple of videos on youtube where people successfully transplant heads and/or platters, but since they have experience, it looks much easier than it is. – PlasmaHH May 21 '17 at 21:40
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    And of course there are absolutely NO "fake videos" of ANYTHING on YouTube - everybody knows that! :) – alephzero May 21 '17 at 22:46
  • To what extent have tolerances shrunk, and to what extent have closed-loop servo systems improved so as to render tolerances that used to be critical, irrelevant? By my understanding, track spacings have narrowed to the point that it would be extremely expensive to ensure that the "wobble" of the disk could never exceed a quarter track, but head-control systems can continuously adjust the position of the head even in the presence of wobble. – supercat May 22 '17 at 5:04
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    @supercat As far as I know, the servo controls in the drive only deal with "horizontal" track positioning. The "vertical" aspect -- head flying height -- is primarily managed through passive aerodynamics. – duskwuff -inactive- May 22 '17 at 5:29
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    @duskwuff: True, flying height is controlled through mechanics rather than electronics, but by my understanding it's designed to self-regulate to a tolerance finer than that of the head's mounting. In older hard drives, if the head came disconnected from the stepper motor it could be very hard to achieve the same mounting with enough accuracy to make the disk readable, but on modern drives the use of embedded alignment information recorded on the disk makes that a non-issue. – supercat May 22 '17 at 5:37

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