[BetterRed]

@Tanjamuse - a file sync program operates on the folders and files in the library, that includes the database (metadata.db) - its not aware of what's in them.

Essentially the file sync program will maintain a copy of the library, it will keep it up to date by detecting additions, deletions & changes. The first time you run it the entire library folder will be copied from your laptop hard disk to the usb drive, this will take a while (possibly hours) depending on the size of your library (in disk space terms). After that only the adds, deletions and changes will be actioned - so it will be much quicker - typically a few minutes.

As eschwartz suggests with FFS you would use Mirror option. The 'beauty' of using a folder and file synchronisation approach is that the when something goes wrong and you need to recover a file, the library on your backup drive looks exactly like it does on your laptop, and you can use your 'normal' file manager to copy files and folders etc - you could also use it as a calibre library in its own right.

You haven't told us what sort of laptop - Windows, Apple Mac or Linux.

BR