[BetterRed]

@robin58 - Strange, I would have thought the two lines quoted would have been amongst those least likely to change.

But why are the files changing? There are things that change the book metadata that are not immediately obvious, such as - adding, changing (the definition) and removing custom columns; changing the case or spelling of an author, publisher, series, tag value etc,

The opf files are there so that the calibre database (metadata.db) can be rebuilt if and when necessary. If you don't back them up, then in the event of a disaster you may not be able to rebuild your library... easily.

You may have to add all the books again... that should fairly easy. You may have to put the metadata back together again... that might be hard if it was created manually and it includes custom columns that are not stored in the books formal metadata. I add a belt to my backup braces - all the metadata is in the ePUB's book jacket.

How frequently do you do the backup ? What software do you use? And when you say 'online backup' does that mean over a local network, or over a wide area network?

BR