If you've never taken any steps to fix the "issue" and clicked cancel every time the DNS-323 prompts you to fix it, then the problem WILL remain.
I would start by backing up the data - there is nothing to indicate which drive the DNS-323 thinks is defective, and you could have it format the wrong drive and sync from the other 'older' one, causing a loss of all data stored after the "degradation" occurred.
In an ideal scenario the unit - which seems to think one of the drives is new - will format the "new" drive and then sync from the other drive, but, better to backup whilst you can and be safe, rather than sorry.