HDD had a head crash, so I took an almost identical HDD and substituted the head assembly from the good one into the failed one (just as an experiment). However, I get the same result as with the failed head: the substitute head knocks of and hits against the middle cylinder (where the platters sit). I took the substitute head and put it back into its original (second) HDD. It started doing the same thing (though worked before). Does it mean that I have likely damaged the "good" head assembly, while handling it or by putting it in the bad HDD? Or that the problem is not with the head assembly at the first place?
Thanks for any replies in advance

Likely HSA was killed either by you or by bad drive

If it was the drive the most probable cause would be media damage or bad PCB

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

I am pretty sure PCB is OK. There is media damage though (when the head crashed, it left a typical scratched out circle on the platter surface). Does it pretty much mean there is no point trying to restore the data. I only wanted to look what the HDD has stored on it (partition table, for example)? Is there anything that can be done at all?

It all depends om the drive's model and amount of money you willing to spend

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

Why can't I post to this thread directly? Why do we need a moderator? Am I violating some rules?

Is DIY possible? What magnitude of dollars are we talking about both DIY and non-DIY (at cheapest)?

Btw, the drive is Seagate ST3500320AS (500GB)

several thousand dollars for partial recovery and only some companies would even try to recover data. There is also a big chance that recovery will not be possible at all (depends on media damage location)
I don't think DIY is possible, because this type of recovery requires lots of practice and deep understanding of drive internals

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

Ok, understand.
Thanks for the reply (and for the private one, too).
Chucking the HDD into the bin

You could sell the PCB on eBay.

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A backup a day keeps DR away.

I hope mine doesnt have a head crash