My D7000 quit responding with my Sigma 17-50/f2.8 attached. The lens will auto focus but the shutter will not work. I switch to manual focus and after focusing the shutter will work. So, I put the Sigma lens on my D300s. It never fails to work set to auto focus. After that I try 3 other lenses on the D7000. It never fails to work set to auto focus. I put the Sigma back on the D7000 and it intermittently works and fails. I can't conclude it's the lens because the Sigma works perfectly on the D300s. I can't conclude it's the camera because none of the other lenses fail to work on the D7000. Any theories?