[Adoby]

While calibre is running it assumes that it has full unrestricted access to the contents in the calibre library. That means that no other programs can access the library at the same time. If this isn't true, then problems may ensue.

Also any changes to the contents inside the calibre library, done manually or with some other software other than calibre, can (and probably will) cause problems and data loss. You are strongly advised to not even look up the skirts of the calibre library, and if you look at least don't touch anything. Use the GUI.

When you backup your calibre library, do so when calibre is not running. Make sure that you backup the complete library as a single unit, to preserve the internal structure and function.