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Matt, you are completely right about functionality of USB ports on control surfaces, they are intended to connect external mouse/keyboard/touchscreen/flash drives. Apart from already mentioned EtherSound based Digigram PC interface, you can use a number of MADI interfaces such as RME HDSPe MADI or MADIface. This will give you 64×64 channels of recording/playback. All you need is to install M-MADI-A in Port B of iDR-16/iDR-32/iDR-48.