If you add a USB flash drive to the correct USB port, ie not the audio one. Then login to the web browser at myinnuos.com there is an option to configure backups. You need to enable the USB option here.
This will format and completely wipe the USB. Sorry but I don’t know how to do it as a partition without completely wiping the USB. Perhaps someone else can help with that.
Now if you login to Roon and navigate to Backups when you select location it should see the USB as “Zenith backup drive” or something like that.
Cheers,
Alan