[AnotherCat]

FreeFileSync, for example, gives the option of using the Windows Shadow Copy Service which should be quite safe for backups (i.e. a one way update or one way mirror to an archive target, not a two way sync) of Calibre when open (and email and other databases too); in any event I have never struck a problem with it.

I am not promoting FreefileSync in saying that, it is just the one I have used for a number of years now, I have no doubt there are other alternatives. However, as far as I recall SyncToy does not use the Shadow Service (and there will be others too, so I am not picking on that; again it just the one I used to use) and the warning given by others applies to updating live databases (SyncToy's original purpose was for simple backups of the likes of directories of photographic images and its functionality never really developed beyond that type of purpose).