Internal SD periodically becomes inaccessible
I had the problem with my Ainol Novo 7 Crystal Quad Core and the solution was simple but took hours of research.
The internal SD partition that goes beyond the memory reserved for the OS and some apps may have become corrupted.
Go to the backup/reset tab in "settings" and click the erase SD card.
You will be told that you will lose all the information on it but you will only lose that which you cannot access anyway.
Erase the card.
Once it is erased, you will be able to mount it when you go to "storage" and you push the remount button.
Hope this of help and use to you. It worked for me.