Hey, I’d recommend taking a look at Syncthing. It’s software that runs locally, and you can configure the synchronisation to occur in a unidirectional flow instead of bidirectional. I.e. configure it to only synchronise data from the source drive to the backup drive.
Hope this helps and all the best!
EDIT: As pointed out, Syncthing is more suited to syncing data between two or more devices so may not be the right solution for OPs use case.
Hey… Ah, you are correct. Thanks, I’ll update my original comment.