iMac 27" mid 2010 now running Mountain Lion wish to create dual boot Snow Leopard and Mountain Lion to run older scanner and other peripherals. Original OS X install disc gives error message "you can't use this version you have 23.1.1 Created time machine backup. Created bootable Mountain Lion USB Created bootable Snow Leopard USB Booted iMac from Mountain Lion USB and erased HD Attempted to boot from original start up disc but it hangs on Apple logo screen Attempted to boot from Snow Leopard USB and hangs at same point Attempted to boot from Snow Leopard retail disc and hangs at same point. Boots every time from ML USB When I boot holding OPTION I can select from 3 different start up discs, but only ML gets past the Apple logo screen. What is the reason why the iMac, which shipped from memory with Snow Leopard, will now not start up from SL even when the HD has been erased?