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I figured it out -- you have the LOAD LOCK switch set to LOCK. For whatever reason, when you try to load a track from USB if connected to HID, it doesn't warn you, nor does it load the track even if playback is stopped. But if you toggle it to UNLOCK, you can load a track from USB.
I'll pass this to the engineers - not sure if it affects other models, or if they'll update it, but at least we know the work-around.