@Paul_Lewis, can you copy the RoonBackups directory from your Dropbox folder to your external SSD? If so, then go into Roon’s Settings → Backups and with the external SSD connected to your Core or on your network, select Backup now and then Select location… and select your external SSD location with Roonbackups, then backup to the external drive. Roon now backup to your external drive.
You can confirm this by checking the path for your scheduled backups and verify that your external SSD is the default path.