I found an excellent post that I used on my DS209J when the volume crashed with no other log information.
FYI, I've replaced with commands in that blog post with the correct ones for a 2-bay NAS where the volume is 'md2'. Login to your NAS with SSH (after enabling that feature under 'Terminal') and use root for the username and the admin password for the password.
echo check > /sys/block/md2/md/sync_action
If you login to your NAS dashboard after you run this command, you'll see the following.
To scrub and fix the errors, run this command:
echo repair > /sys/block/md2/md/sync_action
If you check the dashboard again under 'Storage Manager' , you should see this screen.
Hopefully it reports back