I had a similar issue. In display preferences, is the main monitor (i.e., the one that has the menu bar) being displayed from the mini display port or from the USB device? Make sure it's the mini display port that has the menu bar by going to the device preferences --> appearance tab --> and drag the menu bar in the images to the monitor that is being run by the display port.
I have a different chipset so maybe it's a different issue. But give it a try.
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