I just got an external drive for my iMac (desktop) and am trying to steer my Time Machine to it. The external drive is plugged into the USB port and its light is on, and I am able to get onto the internet so I know my wireless is working (my ethernet cable hasn't worked in years), but when I try to direct my Time Machine to back up onto the new external drive, I get "AirPort Utility was unable to find any AirPort wireless devices" etc. The "manual setup" button is greyed out. What am I doing wrong?