Okay, for the sake of simplicity, I'm going to say remove all your USB devices, cause I don't know how many you have. Reboot. Then try mounting it. If it still insists on not mounting then two things... First off, are you using a Pandora's memory stick? If so try with a normal one, as maybe the offset blocks on the Pandora's memory stick may cause issues, but it never has for me. If that's not the case then try the former answer yet once more, and if that doesn't work I'll try and think of something.