[Sirtel]

First, don't put your Calibre library anywhere that automatically syncs, shares or makes backups to somewhere else. Put it in a strictly local place on your local drive and use FreeFileSync to make a backup copy. A backup copy can reside in a cloud, on a networked drive and/or on an external drive.

And yes, FreeFileSync detects the differences between the original and the copy and only updates what has been changed. Only use the mirror mode, never two-way for your calibre library.