I've been hoping someone with more experience with the stock firmware on these devices would chime in. If you search previous posts on this forum you'll likely find folks who have had similar issues in the past.
The best way to proceed depends on a few things, most importantly whether you need any of the data off the device. If not I would remove the drives, wipe them, and run diagnostics on them to determine if one or more of them have developed bad sectors or other issues. Then re-install the firmware or Debian.
Obviously, If you do need data from the device you'll want to focus on recovering data before doing anything like that.