I sent you a private message this morning. Hopefully, it helps.

We are old school and rarely use the USB connection other than to access the internal drives to manage files, although you can use it for MIDI data. Are you using the USB port in the rear or on the sides of the unit?

In our case, we are using the MIDI in/Out (DIN) connections to a Focusrite Audio/MIDI interface. That interface is a USB connection to our Mac.

Couple of things to consider:
There are 32 MIDI channels to work with.
MIDI In1 has access to the first 16 channels and what Ketron calls the GM Sounds.
You can manage the transmit and receive of those channels via the "GM Part" button.
MIDI In2 has access to the second set of MIDI channels.
You can manage the transmit and receive of those channels via the "Arranger Keyboard" button.
In2 also allows access to both the User Voice and User Style banks in addition to the Voice Presets.