I was tired that night, I plugged in an incompatible exterior hard drive to my iMAC, one that is compatible with Windows Vista (brand new and unused). Since that night, my iMAC has frozen all the programs including FINDER. The only thing showing up is Skype but it is not working...FORCE QUIT is not working either. I rebooted three times but no change. What can I do? Humbled and ashamed.