[Adoby]

I believe this is what happens:

You add a book to Calibre, and some initial values are set for Author and Title. Either from existing metadata or from the file name. The book is stored in a folder named after author and title. Like this:

Calibre Library/{authors1}/{title1} ({id})/{title1} - {authors1}.epub

The contents of the file has not changed. Checksum unchanged.

After a metadata download this may change to:

Calibre Library/{authors2}/{title2} ({id})/{title2} - {authors2}.epub

But still no change to the metadata inside the file. If you at this stage peek inside the Calibre Library and manually retrieve the book, you will find that it has been renamed, but the metadata inside the book has not been changed, so the checksum should be intact. The new metadata is saved by Calibre in a separate database file, metadata.db, and it is this information that is shown in the GUI of Calibre. But the metadata embedded inside the book file may have completely different values to those shown by Calibre.

It is considered a bad(tm) thing to peek and rummage around inside the Calibre Library like this... People usually end up with missing books, corrupt database or other problems. There are stickies here about the dangers. But it *is* possible.

Usually you write the books, with the updated metadata to disc, or send to device. During this process the old metadata inside the file is updated. This is when the checksum is changed. At the same time you can change the filename that is used.