If you haven't made a backup of the data to another location, now would be a good time.
Afterwards, shutdown the DNS device and test each HDD using the Seagate Diagnostic (seatools) utility - you'll likely need to do this from a pc. Make sure to insert the HDD's back into the same DNS slot you took it out of.
If both HDD's check fine with the seatools, I would suggest removing the partitions (again using a pc) and rebuilding the RAID1. Before copying data to the unit, I would wait the couple of days to see if they degrade.
HTH,