I have an old Imac that has some issues and i've decided to reinstall SL from a disc image on USB (the same one I used once before) and for some reason when I try to restart and hold down the option key, it will bring up the HD and the USB with the SL dmg file but as the reinstall is just about to start i get the error message saying that it can't be installed on the MAC HD.... i'm not sure what is going wrong. Can anybody please help? I don't have the install discs and that's why I'm going from the USB. FWIW it has all of the SL updates up to Yosemite, not sure if that is an issue. Any assistance at all is greatly appreciated.