I had been moving files back and forth between my WD mybook USB External HD when I pulled it from the iMac without ejecting and plugged it into my air. The Air's screen dimmed and this popup came up. Now everytime I plug something in to my USB port, i.e. USB jump drive, External HD, even my External Air DVD drive, the same thing happens, screen dims and this message pops up. THe only item that it has not frozen up on is my Iphone, that connected fine without crashing.