Hey guys, yesterday I upgraded my MBA (late 2010) from beta 3 to beta 4. After the required restart the Apple logo shows up but then everything freezes. I did several force shutdowns, but the MBA does not start anymore. Not even a progress bar shows up. I tried to start the recover-modus (apple-key + R). Indeed, a progress bar appears, but it stopped at about 60%. It freezes again... I have no idea what to do next... Any hints?