My +19 to 24V supply is a lab power supply, which has red(+), black(-), and green(chassis ground) terminals. My understanding is, since I'm connecting through red and black, I don't think the earth ground is part of the ground connection. Anyway, I did try tying +19 to 24V directly to the motors bypassing the FFB USB control board as you suggested and I still got the same USB issue. Looking closely at my build, I did notice the USB panel-mount connector that I got through Aliexpress does not have the shield tied to ground. I don't think it would matter but I would try to connect it to ground just to be sure.