I just picked up a used mid-2011 27" iMac that has Snow Leopard installed. What are my options for upgrading to Mountain Lion? That is, do I have to simply purchase the $20 ML upgrade and apply to Snow Leopard, or is there a way that I can wipe the drive and install ML from scratch? I would prefer the latter option, rather than upgrading from someone else's system, with whatever they may have had on it or done to it. It is my understanding that ML is only available online. If I can install to a blank hard drive, how do I go about doing this? Do I create a separate bootable drive with ML install on it? Thanks.