Creating configuration files is known to be buggy - but saving over the top of an existing file should be working. Can you describe what happens when you try, please?
It seems I can change and save settings if I save to the default config file on both my Sandisk cards. So I must be remembering wrong then. I was sure I had problems even saving to the default config on one of my Sandisk cards. If I try to save to any of the numbered slots I just get an error saying it cannot create the config file.