Hello,
For someone that ever hit this situation :
I have been able to read disks using this article :
https://www.synology.com/en-global/knowledgebase/DSM/tutorial/Storage/How_can_I_recover_data_from_my_DiskStation_using_a_PC
I have rewrote the usb stick with same image as initial ! because board it's old, cpu it's amd athlon and nic chip it's realtek, I needed to use a special lzma...
After doing this, tries to recover and blocks. Dead end...
I think that those disks are irecoverable. USed to be a SHR composed of 3 HDD's. IT seems that only one it's valid now. One has a lot of alarms on SMART and the third cannot be seen even at boot time.
Thank you for helping me !
Daniel