Houston, We have a problem

I have one 2 TB HDD in Q10 pro and one external 1.5 TB by USB. They are full of 1080p movies. In Kodi library, there are 370 movies. When I scroll down the movie list and press one of the movies, it starts to play. Some movies are in the internal HDD, some are in the external HDD. They are all combined at Kodi library. It generally works fine. Sometimes I have an error as "the file does not exist in library any more, Do you like to delete the file?" I do power off and then on, then kodi recognizes the HDD and plays the movie file. Last night it happened again. I powered off and on 3 times but same problem continued. At the same time, I can play the files without Kodi library, by using file browser.
What can be the reason that Kodi library sometimes does not recognizes HDD?
P.S. I have 2.05 android 7 and kodi 17.4. It happened even with the previous firmwares and Kodi 16. No change.