My '08 iMac came with Leopard which I upgraded to S.L. then later upgraded to Lion by download. A while ago I started with screen freezes,pinwheels and black screens, sometimes 6 times a day. I want to wipe the drive and reinstall Lion. I tried to reboot and reinstall but I get the "unavailable screen" as described by others. I have changed used ID and passwords over the years, so that may be the problem. Is Lion still available on disk? Should I do this or go for a later OS? Will my TM backed up data be restored to a later OS?