This can happen when powering the C600 with the USB bus.

While it is possible to power the Fast Track C600 through the USB cable without the power supply, inherent power limitations of the USB bus make it possible to enable inputs 1-2 and outs 1-4.  Analog inputs 3-4, analog outputs 5-6, and headphone output 2 will not be available. These inputs and outputs are only available when the external power supply is used.