Thanks Kovid, but the database in on a local external USB drive over which I have full read/write privileges and my Mac is without any 3rd party antivirus.
Could you maybe suggest some other leads that I could investigate?
Also, if I can't find anything and must reimport all the books into a fresh library (losing, I suppose, all my metadata in the process), should I simply go to 'Add book... (assumes every ebook file is a different book)', select the folder of the old library and just let Calibre work for a few hours? Is there something else that I need to be aware of?
Thanks a lot! W.