Well that’s very interesting. I don’t have a need (currently) for this, however very useful to know that this functionality exists. I imagine this may solve the issue @Thorsten_Heinrich is having, however.
The hold up is that with an internal disk, Roon treats it ALL as watched. you do not have the option of segmenting sections off as not watched. That is why with an external drive, you can both hold Watched Music folders, and non-watched backup locations. Unless you are backing up to a network location, you will have to have an external drive plugged in to backup anyway.