This happened to me 2 days back. I reboot the NAS and everything worked fine. Today, I could not see any files on my 2nd drive. (I have setup RAID as JBOD). I checked under storage - volumes and raid management,it said could not mount drive. I reboot the NAS, and it show the files in that drive again. After few mintues, the 2nd drive could not be accessed at all.