on request .. here some more details + image:
The HDD was usually mounted in an external case (icybox), which was usually made for two HDD and worked well there. After we mounted a second one - everything seemed fine. But when I looked at the case some hours later, the whole case had stopped working. It took me a while to notice, that when I remove this HDD, which is the subject here, the case inclusive the other HDD started up again.
The removed HDD though just did not start up at all. Bringing it to a single external case only set the power control lights of this case to flickering - thus we assumed, that there is an issue with the electronic circuit.
Meanwhile, as mentioned before, I got the HDD starting, when I had swapped the PCB from an elder model of the same SAMSUNG HDD HD753LJ ... but I did not get it recognized .....
... so that's basically the story .. I also enclose an image .. I hope it's good enough - as I only have my mobile around at the moment ....

Thank you for your immediate reply.