which might be important if a local disaster happens and takes out the only backup on the on-site media.
A variation on the WinSCP idea is rclone SFTP (or some other protocol). This only propagates changes.
Questions about destinations, rclone, scripts and jobs talks about this and other possibly relevant topics.
If you’re sure you want this, the basic need is to keep local database correctly describing the destination, because database says what has been backed up, and this is how the later backups can write less data.
Although it’s possible to create backup jobs in pairs, and figure out which job goes with the current drive,
was an experiment which possibly loses a little performance but gives you lots of leeway on plugging in a drive and being sure that the database matches the destination files – and only one job has to be created.
Be sure you don’t unplug a drive in the middle of a backup – another problem the network method avoids.