I had some conflict while updating the kernel and I had to re-install.
Long story short, I had to burn the iso image of manjaro-architect and I accidentaly did it on the external HDD where my backup was.
The HDD is 5 Tb, of which 3.5 were used, thus the image didn't overwrite but a tiny fraction of them.
I used testdisk to check and the data are still there, but cannot be restored, only copied to another memory, that I don't have.
I tried to force the restoration following these instructions: https://www.cyberciti.biz/tips/surviving-a-linux-filesystem-failures.html.
Unfortunately there seems to be a problem with the filesystems.
Any suggestion would be appreciated.