[robin58]

How do you intend on excluding them in an automated manner??

I was simply going to exclude all opf files. Both Goodsync which I had been using and FreeFileSync which I have also been trying recently allow this fairly simply.

A tag was edited most likely, depending on where this (and other actions) was done, this will cause all books sharing that metadata to be backed up. Nothing *changed* except for the timestamp usually, and apparently a new piece of info calibre decided to store by default in the OPF, which was not previously present and thus had to be added.

I have no tags. They had become a mess a while back and I simply removed all tags from all books.

Changing/deleting any tag used in a book:
The (backup) OPF does get rewritten because Tags are stored in clear text.

OPF files are small

See my comment above. I already had deleted all tags from every book I had.

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, which should fairly easy, and put the metadata back together again, might be hard if it was created manually and it includes custom columns that are not stored in the books metadata.

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?

The change is what puzzled me too. Today I synced everything before the update. I had 3,000+ opf files that were supposedly different and 2 books that I had added during the week. I updated my backup. Then proceeded to update to the latest version of Calibre (1.2.4) and added another book. I did nothing else. Shut Calibre down. And then for the heck of it ran the filesync programs again. I had 549 books that reported changes (incidentally both Goodsyn and FreeFileSync reported the exact same differences). That's when I took the peek inside and only those two references I quoted in my first post above were in any way different.

I understand they're an important factor in restoring should something become corrupt, but unless I'm missing something if the version number of Calibre is going to prevent an accurate restoration, then having a backup is useless anyways, right?

As for frequency, I back it up once a week to a networked drive which then is also mirrored on an online storage as it is a watched folder.