Guess I should have asked, but I just assumed if you had the PC turned OFF, and the board still ran, that you were externally powering the board. None of my machines will provide power to a USB port when turned off.

Sounds like you're plugged into a self-powered USB hub? If that's the case, then this approach obviously isn't going to do you any good.

If you can't use another power source, then I suspect you'll need to dig into the datasheet and find a flag bit to help indicate USB activity.

Can't offer you much help there since I've never done it yet.