Maybe Toucan or some other Windows or Linux backup software can do this? Syncing two drives offline once a month.

So I haven't found the clear answer in Toucan's documentation (where is it at all? never mind, I understand not all software projects place as much emphasis on their documentation as their code as some) to this functionality. Here's what I'm trying to accomplish: I backup my stuff on two external drives, A and B. I store one of the drives off site, at a friend of family member living separately. Let's say every month we meet with the friend and family member and I refresh the content on the drive to mirror the content of the other drive that is with me on which I have changed some content.

Let me explain in more detail to make it clear, hopefully. So drive B is with the friend for a month. The content type I back up on both drives are mostly media if that's important, but I don't think so. Meanwhile I put some new data on drive A which is with me while removing other content, so there is some delta or difference starting to develop between the two drives during the month: Some content remains the same on both while drive A which I actively use during the month get some more data while also some of the data is being removed from it. But most of the data remains the same. So after a month when I meet the friend to sync the two drives ideally I just want to sync the delta, not wipe the whole drive B and write all content to it over from drive A. Just to apply the changes of the smaller delta: To remove the content that has been removed from drive A during the month and similarly add the content that has been added to drive A during the month. I hope what I want to say is more or less clear.

I'm not sure if I do my backup the right way (well, any kind of backup is better than no backup I guess) and if there is a popular free software on Windows or Linux capable of what want to accomplish. If I understand correctly, Toucan is written by PortableApps founder John, and probably isn't widely used by folks who don't use PortableApps anyway. Thanks for reading so far!