Thank you for your answer. I have a second identical device that works. I used electronics from a disk that does not work with a disk from a working device. Unfortunately, the experiment ended in fiasco. I have used the same connections (lan cable, port on the router) which the hardware works. I have no more ideas. Is it possible to connect to the electronics itself and upload new firmware? There is a connector on the PCB that looks like a service.
Assuming you mean the board from the MyCloud, and NOT the HDD driver PCB, then you need to check that they are genuinely identical (and not just both ‘MyCloud’ devices). They must be the same generation of the MyCloud (there are two generations, and they are substantially different, and the firmwares are not compatible).
If you have tow identical boards, and you take the board from the working device, and connect it to the HDD of the non-working device, if that combination does not work, then there is something wrong with either the HDD, or with the image you have put onto it.
There are many threads on how to ‘unbrick’ MyCloud devices, and how to replace an HDD. You need to know if you have a Gen (v4 firmware) or Gen2 (v2 firmware).