To be able to get help on this please describe the following:
(1) How is your Yamaha connected to Cubase? Through a USB cable?
(2) Do you use the Yamaha as a midi controller on Cubase?
It’s a strange process you are describing. If you have a DAW like Cubase and then the Yamaha, why do you need to import anything to Cubase using a USB stick? It basically is counterintuitive. You are using a very long and arduous process which obviously isnt working. Please use the following procedure:
(1) Make sure your Yamaha is connected to the Cubase using a USB cable;
(2) Download and install the Yamaha Steinberg USB Driver
(3) Check the YAMAHA SX900 manual for a guide on how to connect to a DAW How to setup USB MIDI on Yamaha Keyboard and Cubase 10.5 LE - complete MIDI tutorial and midi looper - YouTube
(4) Create a midi track on Cubase and check to see if in the inspector, Yamaha SX900 inputs and outputs are indicated/recognized.
(5) Click on the monitor/speaker icon on the track and press any of the keys on the keyboard for sound.
(6) If sound is noticed, record-arm the track and play a couple of bars.
(7) You might not be able to bounce the midi track to audio like any VST instrument within the DAW. You might need to create and audio track underneath the midi track, record-arm the audio track and hit the space bar to record. The data on the midi track is recorded to the audio track.
I own Yamaha MOXF8 and that’s how I use it for now. I have posted an extensive guide on the various ways of connecting the MOXF8 and using it in Cubase. This might apply to you as well. [An Amateur Guide To Connecting And Using the Moxf8 in your Daw - Cubase - Steinberg Forums](https://forums.steinberg.net/t/an-amateur-guide-to-connecting-and-using-the-moxf8-in-your-daw/14667