I was originally going to just copy all my files on to an external HDD and then download and install Mountain Lion. If everything worked, then great. If there was any problems, I'd have all my important files, and then I have my disks at the read to reinstall any apps. However, I've been reading articles, and they recommend using Time Machine, back up my Mac that way on to my external HDD, every time I plug in that external HDD, would it work as a bootable disk, sort of? Like, I plug it in, and it thinks I want to install Lion? I don't really get Time Machine. Does it back up everything to a folder or what?