OK I solved my issue !
What I performed :
- detach my 2 RDM disks from Xpenology VM
- attach and mount these 2 RDM disks to Ubuntu VM
- try several Unix command line tools (fdisk, e2fsck) : no success
- try testdisk tool : success
With testdisk I succeed in rebuilding one (not both) of my 2 RDM disk partitions.
All my data were present and intact !
As far as I saw, on RAID 1 volume, Synology creates 3 partitions :
- Linux boot
- Linux Swap
- Linux Data partition (the one I retrieved my data from)
Even now, I do not understand what I did wrong in my Xpenology update
One thing I am sure now, is that I will invest in one offline backup hard disk (and investigate as well for free storage on cloud like 2Tb at BAIDU) to prevent any problem like that.