Hmm...not sure exactly what you're asking, but the RME does NOT draw it's operating power from attached USB sources.  The RME is self-powered.

Be careful here!

Curt

The term "VBUS handshake" is not an official term. There are two things that may fall under this name. According to the USB spec a device (such as a DAC) cannot connect to the D+,D- pins unless the VBUS pin is near 5V. Some DACs power some or parts of their circuitry from the VBUS pin, but even if they are self powered they are still supposed to look at the VBUS and not connect if VBUS is less than 4.75V or so. SOME DACs completely ignore VBUS and connect whether VBUS is powered or not, this is not spec compliant.

There is another use of VBUS, for OTG ports (which can be either host or device) under some circumstances VBUS is actually used as a communications line, the voltage is pulsed, this is used to resolve which type of functionality the port should take on. This is rarely if ever used with DACs, thus "VBUS handshake" usually refers to the above, the device should not connect if VBUS is not near 5V.

This all assumes that the ground line is always connected. Things get really wonky when people try and disconnect the ground line. Some aspects of the USB signaling will not work without the ground line, and the VBUS detect will also not work without the ground line. There are a few very specific instances where some people have gotten away with cutting the ground wire, but in these cases there has ALWAYS been some other path where the computer ground gets connected to the USB receiver chip's ground.

Easy enough to test...

You probably have an excess of USB cables. Choose one, cut it open, cut the 5V wire and check if you ADI-2 Pro or DAC still works.

You can later use that cable for measurements, or injecting 5V from a PSU to a bus-powered device.