It may be curious to question how a peripheral device could draw more power than the source could supply.
The USB port could only supply up to 5v (or any other maximum value). So, when the error message says that the USB device is drawing too much power, does it mean that the D7 is drawing more than 5v from a USB port that could only supply up to 5v power?
It would only be possible only if the D7 (or any USB device) has the ability to reach into the USB port on our computer and suck out (by an act of force) more power from other components and channel them into the USB port.
A better way to make sense off the error message is that the 5V output from our USB port is insufficient to power the D7 fully which might require more than 5V power. Thus, the computer may disable other USB devices just to ensure that a stable 5V of full power is being supplied.
But still, that doesn't make sense because all the other devices would still continue to work normally, including the D7.
Actually it has nothing to do with voltage. It is the current that matters. In electricity, power is the product of voltage and current and is given in watts. If the D7 draws 0.5A at 5v, then it uses 2.5 watts. The USB device cannot change the voltage that is supplied to it, but it can pull as much power as it wants. Unless the USB hubs can actively limit the current they transport.
USB divides power as "unit load", each having 100mA in USB 2.0. For example, if the D7 draws 500mA, then it uses 5 "unit load". The problem is, any USB device, when originally connected, is given only 1 unit load, or 100mA. It can then ask the root hub for more units and the root hub will allow it more if it can.
It's not easy to build a USB device than can manage this properly. To work with only 100mA, the D7 would have to shut down it's amp and as much features as possible. Class A amps are not known for their power efficiency. I don't know if a root usb HUB can actually limit the current it supplies to any given USB device, and I would be very surprised if it did. That said, if the D7 doesn't care about the unit load system and the rule that at start-up, any USB device must draw no more than 100mA, then it might explain why the OS complains that the D7 draws too much power. It shouldn't be a problem, as long as the D7 stays under 0.5 amps.