You can have a backup copy of your library in Dropbox. You should not have the copy you are opening with calibre in cloud storage. It's not a matter of if you will have problems, it's
when you will have problems.
The safest procedure going by many posts in these forums:
- Keep your library in local storage syncing to the cloud
- Stop syncing your calibre library
- Open calibre, make your changes and close calibre
- Restart syncing your library.