I found at home a Toshiba hard drive MK4018GAP and get an idea to learn something and use this hard drive as an exercise.
When I connected to a PC you can hear clicking noise in every 4 seconds. It's not recognaizable by BIOS. I have already opened the cover, heads are in place and platters spin freelly manually.

If anyone is interested in this ecercise feel free to send any message in this topic.

what can we talk in this topic?
make focus in the same point & reading data by eyes?(joking, don't angry!)

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Bad heads probably.

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if that would be bad heads, what about not recognaizable by BIOS? I'm not a profesionalist in this but I would rather think about PCB. How PC get info about hard drive? Read some data from one of the chip on PCB? Probably it making self check on the start, but would it matter to send some info about hard drive to PC when will fail self check?

Its not recognzible by BIOS because BIOS use a high level to comunicate with HDD (Int13h), if BIOS doesnt got answer from this way, said "there its not hdd here", but the HDD its comunicating to another low level, by ATA protocol, and showing ATA registers, search about that

Regards