I should add that the problem got worse after the El Capitan 10.11.2 update. Prior to that update Calibre could occasionally open the database, but after the 10.11.2 update I've not yet been able to get Calibre to open the database.
I should add that the problem got worse after the El Capitan 10.11.2 update. Prior to that update Calibre could occasionally open the database, but after the 10.11.2 update I've not yet been able to get Calibre to open the database.