For those in need
Boot the syno and hit reinstall
don't update to the latest dsm during the install ... choose to upload a file (then take a 5.1)
Update without loosing the settings
And voila
Everything is fine, juste a little update for 5.1.5022 updt5
Thanks
How do you access the partition with grub ?
If the Virtual Diskstation is booted ... you have only access via winscp to the Synology structure ... no the startup disk one