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I have created and updated many of my catalogs, however got an error msg recently. It says: Catalog already in use. I'm quite sure LR is not open, dunno what to do.
Another thing to look for is the presence of a catalogname.lrcat.lock file alongside your catalog (.lrcat) file. If LR is not running, this file should not exist, and can be safely deleted. I believe this is an ancillary file created by the SQLite engine which powers LR.