I need to do what the message says without cubase thinking I have connected a second keyboard.
I need cubase (and/or Windows 7 64 bit) to forget the initial settings.
What to do?

I simply don’t understand your question but I may be wrong.

Unfortunately Windows enumerates any devices connected to USB ports automatically. Connecting a device like the MOXF to a certain PC USB port will not create the same internal USB port definition/name in Windows registry as connecting the same device to another USB port. After installing the Yamaha Steinberg USB Driver the very first time, it’s always a good idea to remember which USB port has been initially used for that connectivity. My personal advice would be to retain that initial setup and always connect your devices to the same USB ports exclusively.

I’m currently not at my gear, but as far as I remember your very first MOXF port connection will be defined in Windows by a name like “MOXF...”, ending with a tag like “...1” (Edit: or NO tag at all). If you then plug the same synth into another USB port, this port connection will be created internally with tag “...2” and so on. Now, if you have changed the physical connectivity for some reason and you are starting up your Cubase system which is expecting to find your MOXF connected at a certain USB port, there might be troubles in the MIDI setup as you already pointed out.

You now have the option to open your Cubase setup and change the related MIDI port settings to reflect the “new” USB port name definition for your MOXF. Or - more drastically - uninstall and reinstall the Yamaha Steinberg USB driver which will also remove any previous USB registry entries related to your Yamaha gear. Then you can start from scratch, plugging each single device into its dedicated USB port which you should use from now on exclusively. After doing this and after setting up your application’s MIDI/audio parameters again, I’d also recommend to take notes about your final configuration. This is the cleanest way from my perspective to avoid such problems in the future.

There are some “intelligent” applications like Melas Tools which will identify and address the MOXF synth always correctly, regardless to which USB port it is currently connected, always assigning the correct MIDI port names to it as well. However, unfortunalely there are other examples like the KARMA MOTIF application which always expects to reveal a certain “pre-defined” combination of USB devices which always have to be actively plugged in and configured in the same way, as the USB-to-MIDI port assignment seems to be based on numerical-to-alphabetical mappings only, directly rellated to the Windows definitions. If you now occasionly decide to plug in an additional USB controller like a nanoPAD2, the previously defined USB-to-MIDI port mapping will immediately get messed up automatically and you’d have to start all over again by re-defining all ports manually.

The conclusion from my point of view is that you have to find out how your used applications finally deal with USB ports and their related MIDI port definitions/mappings. My recommendation is to retain the physical USB setup in your environment as much as possible, regardless which application you are planning to use at the current time. This will provide a good chance that you won’t have such issues anymore.