Always contact Synology support first for help. If you like to try on your own risk:
after a crashed volume which is not recoverable from the GUI you can try to mount the volume as read-only in a terminal.
Make terminal ssh connection to Synology NAS.
mdadm --detail /dev/<mdX, check cat /proc/mdstat>
check md not listed. For example md9
use at own risk:
mdadm -A -R /dev/<md not listed> /dev/<disk not crashed> /dev/<disk not crashed>
-A, --assemble Assemble a pre-existing array
-R, --run Attempt to start the array even if fewer drives were given than are needed for a full array. Normally if not all drives are found and --scan is not used, then the array will be assembled but not started. With --run an attempt will be made to start it
command should return the message "mdadm: /dev/<md not listed> has been started with x drives."
lvm vgscan : take note of volume group name
vgchange -a y <volume group name, for example:vg1000>
The command to mount the volume as read-only in order:
mount -o ro,noload /dev/<volume group name>/lv /volume1
volume group must exists on your system.
the volume should be reachable in the terminal session.
To see the data in DSM using File Station or Windows File Service (SMB)
synospace --map-file -d