[Adoby]

As I see it you still only maintain your original library/libraries, exactly as usual, but now you also have a backup copy.

I use this procedure, an intermediary copy, to send books to my device and to make backups of the entire calibre library.

I have a folder on my NAS that is a copy of the book folders on my device. I connect to this folder from Calibre and send updates and changes there.

My device automatically sync from this folder using the app FolderSync every hour, but only if charging, or if manually launched.

Also when I quit calibre a backup copy of the calibre library is automatically updated using rsync. This backup copy is then daily backuped using "Time Vault" style backup to external USB harddrive. The backup is also used to automatically refresh the library used by a VM, every time the VM is started. I sometimes use the VM to run the calibre content server.

I use the intermediary buffer to avoid having to turn things on and off at both ends, before I can send/backup.

I only have one "main" instance of calibre, that is where I add books and update metadata and so on. I don't fetch news.