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All metadata for a particular Calibre Library is stored in a single SQLite database with a file name of "metadata.db".
In case the metadata.db becomes corrupted, each Book's specific metadata is (or should be) backed up to a file named "metadata.opf" that resides in the Calibre directory structure for each respective book. You can restore the metadata.db using those individual metadata.opf files via Calibre's tools to do so.
In the same top level folder as metadata.db there is a file named "metadata_db_prefs_backup.json". This is used to restore Library-specific customization options lost because of a metadata.db corruption.
Backing up metadata.db and metadata_db_prefs_backup.json routinely is always an excellent idea.
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