O.K., with RAID 1, you won't need to reconstruct the RAID to make a backup.
Your quickest route to recovery is to mount one of the disks on a Linux machine and make
a backup of your data. Then, place both drives into the DNS-321 and re-setup the RAID1.
Finally, restore your data. And keep a current backup, in case this problem resurfaces.
Since you are Linux savy you can do a google search for 'fun_plug dns' and this will reveal
another forum which may help get the features you want out of your NAS.