How to choose a different storage directory

I looked through previous questions on this matter, but didn't find an answer that applied to me and that I understood. I am only moderately competent in Linux, having forgotten a good deal of what I once knew.

I have version 21 "vanessa" running well on a Dell laptop, with baseline software installed, and I want to create a live iso file so that I can easily return to this point if necessary, or simply load it onto another computer and hit the ground running, as it were.

Systemback runs out of disk space, even though I have not installed much additional software -- VLC, Forticlient, and Dropbox. I excluded dropbox from the backup.

So, I have a blank thumb drive in ext4 filesystem (it had the live Linux that I used to load this laptop). It is mounted at /media/rick/Linux\ Mint., and it has 30 GB available. However, Systemback doesn't allow me to select it, or even any other directory on the local drive, though it obviously meets the writeable Linux filesystem requirement. All the local directories, including /home, are in red with a leading "x".