The pre-amp, as the name implies, is used pre-amplify the the weak signal that is read from the platters, before the signal is sent to the main HDD PCB for further processing. At least that is what I have read in many articles and posts.

Now, my question is why does the HDD click when the pre-amp is faulty ? Even if the pre-amp is faulty, the HDD Write-Element is still functional, is not it ?

Appreciate any explanation.

I'll try to explain it using fuel system in a car as an example.
You have a gas tank (media), fuel pump (heads), fuel injector (preamp) and the engine(PCB)
If you tank is full of gas (data) and the fuel pump is working fine (heads are physically fine) but fuel injector can't deliver gas to the engine (preamp is broken) then the engine won't start (drive will click)

That's if we are talking about reading process only. But the preamp is actually microcontroller it's not only reads or writes data it also controls from which head to read data (data is read one head at a time), what parameters to tune for the selected head and how much current to send for a heat generator (it controls head flying height). So if preamp is bad, PCB couldn't even select a head to read from(through the preamp) on the first place
Clicking is consequence of a failed servo seek process when drive needs to read a servo track to know where heads are but couldn't read and keeps moving head stack until it bumps into the stopper (that make the clicking sound)

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SAN, NAS, RAID, Server, and HDD Data Recovery.

Thanks Doomer.

Datasheets for HDD preamplifiers:
http://www.users.on.net/~fzabkar/HDD/HDD_Preamp.html

Preamp Block Diagrams:
http://malthus.zapto.org/viewtopic.php?f=59&t=229&p=582

In the old days before embedded servos there were actually two preamps -- one for the dedicated servo head, and the other for the read heads. In the latter case the heads were multiplexed. The write circuitry was separate.

Nowadays a single preamp handles everything. That said, it's still conceivable for one of the preamp's functions to fail independently of the remaining ones.

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A backup a day keeps DR away.

Out of interest, did any high perf disk makers every try to drive all the heads on a multiplatter drive simultaneously ?
Obviously needs more preamps and move controller power and logic

really very good simile

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Tawfeek Mokhtar
http://alsafwapc.net
Alsafwa Data Recovery Egypt

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All went well until I plugged the drive in.

Not exactly like that but at least one attempt has been made
The drive was called "Chinook", it has been made by Conner Peripherals. It had two actuators

Reading all heads at the same time is not possible due to inability to position all heads at the same time on the center of a data track on each surface. At least RRO and NRRO is different but on newer drives with VTPI number of track per surface and physical beginning of zones is also different

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SAN, NAS, RAID, Server, and HDD Data Recovery.

http://www.google.com/patents/US5223993

Dual actuator

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All went well until I plugged the drive in.

It would have been possible on those drives that had a dedicated servo surface.

If you have the time, you could look through the old archives ...
http://bitsavers.informatik.uni-stuttgart.de/pdf/

:-)

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A backup a day keeps DR away.

Actually... good analogy, terrible simile ;)

an analogy is about 2 subjects that are similar in characteristics.

A simile is a statement such as "cool as a cucumber" or "quiet as a mouse"

I work in Education, so that's my excuse for the pointless correction