If you're comfie with command line (or the Mac has a good file find that will search everywhere) try a search for metadata.db, the main calibre database that is located in the base directory of your calibre library.
On linux at least I would use the following command to search the entire file system for it
Code:
find / -name metadata.db
If one IS found then check the directory tree shown as containing metadata.db and see what else is found there.