hello all - looking for some help before i make a Genius Bar appointment. When I turn my iMac on it gets stuck on the white screen that appears before the Apple logo - it stays stuck on that screen for several minutes. I've timed it at 3 or 4 minutes at this point. Finally the apple logo appears and it boots all the way through. This started about 2 weeks ago. at first it was only about a minute, but that was still odd. i thought it was something that would pass but its getting worse. As I mentioned above, it now stalls for several minutes. any help? thanks all!