I have a non-retina 2012 15in MacBook Pro with a SSD I installed myself, running Lion 10.7.4. I use my laptop at work, and my work has informed me to not upgrade to ML because it is unlikely my computer would still be compatible with work. I have an external hard-drive I use as a backup for my MacBook Pro. I used carbon copy to make a backup of my MacBook Pro to the external HD. I can boot from the external HD. The external HD has Lion on it, and I can use the external HD as my operating system if I have the need. My question is, can I install ML onto the external HD and use the external HD to test ML, without affecting my MacBook Pro and the SSD currently residing in the MacBook Pro? How would I install ML onto my external HD without affecting the Lion install that already exists on the SSD inside the MacBook Pro? If installing ML onto the external HD would in any way change my MacBook Pro when I boot from the SSD inside the laptop, then I cannot install ML onto the external HD.