My friends got an iMac-2010 at his workplace, currently uses El Capian, he wish to download back to Snow Leopard 10.6.3 (specially, as i've heard once you apple firmware update, it cannot be lower), I think his iMac is 21". The problem is he bought retail Snow Leopard disc (Retail) from Apple store, as he lost his grey discs. He cant boot from disc, because "c" doesn't work....... I thought it may be keyboard issue, as he can't even get to startup manager either(Alt/Option) Within El cap, i asked him go to to system preferences/Startup Manager, and select DVD, click "Restart" and he said all he saw was Apple log, (a long time he waited) and didn't go any further.. Now he has issues getting disc out... Although sometimes he can, the fact is its not booting. I think the root of the problem though is can't he downgrade his 2010-iMac from El Captan to retail version of Snow leopard. He said the disc is defiantly 10.6.3 and white color, a Retails disc,.. As he tried a 'grey disc' from another Mac, which obviously different work due to hardware differences. Since the OEM is designed for the Mac you bought it with. He doesn't use Time Machine either, so he couldn't restore from that. Main reason he wants to downgrade is because Adobe CS6 only works up to Yosemite. I dunno why he wants to go back to SL, but if it can't read disc, then that's the first problem. Possible hardware issues? From Apple's tech specs: His iMac 21.5' shipped wit 10.6.3 so even after upgrading to elcap, he should be able to go back with he disc shouldn't he?