I'm a complete novice and want to upgrade from Snow Leopard 10.6.8 to the current Mountain Lion. I am not using Time Machine so the only back up info I have is my pics, music, docs, etc that I manually make copies of. Should I back up my current system before upgrading? If so, should I back up to a 500 gb usb drive? Do I back up my entire system or just certain essentials? If so, what is vital to back up? And how do I back up either the entire system or just certain parts? Anything else I should do? Thank you.