I am sorry to say mine needed configuring when I got a new box, two drives had issues and were u/s.  I had 4 (two main and one backup of each one) and the two backups were not able to be seen.  The main drives were ok, fortunately.  Got a tech guy in in the end to check it and had to buy two new backup drives to get it going again.

Spark up the drives and just see if they mount ok - as long as you can see all the data- the worst case is re-back up.  If they don't, it's a pain!  You need to get into the Synology software (which sucks) to do and it takes forever to re-back up.

I now have a back up I make monthly of each 'primary' drive.  I just don't trust the NAS enough.   I had WD reds 4Tb in each slot.  It's only for main 'work' data storage, my desktop (Mac) backs up via Time Machine to a portable cheap drive.  If only NAS worked as reliably