The NAS glitched out during a weekly Windows backup and the file system check was still going after two full days. It would get to around 7-9% done and restart. I had to bypass the file system check so I could use the thing again.
Everything seems to check out--no SMART errors, nothing remotely interesting in the logs in FrontView--but this isn't the first time this has happened. I've had it and the drives in it for over six years now so I'm concerned about its health.
You could try doing a backup (and saving the config) and doing a factory default/restore. Though it would be prudent to replace the drives while you were at it.
