Two weeks ago I installed OSX 10.8 on my Mac mini. After that, my system started to hang a lot, so I tried to erase it and install OSX 10.7 again. I used carbon Copy Cloner to make the bootable USB and it can load successfully on another Macbook. However, on my Mac mini, when I press on the option button to select the USB, it loads the white screen with a ban symbol at the center of the screen. After a moment, my machine automatically shuts down. Does anyone know how to solve this problem?

Edit: this is exactly the problem I have now http://forums.macrumors.com/archive/index.php/t-1207168.html

Update 1: the hdd seems ok (from disk utility). I restored the booting partition by disk utility (drag the dmg file to that partition). After restarting and using that partition to boot, prohibited symbol is still there