But how do I do it? I've never bought 10.8 (or .7 or .6..). I have a file called "OS X 10.8 Mountain Lion.dmg" that's 4.35GB, but I don't fully trust that and don't wanna risk screwing something up. I just got this a couple days ago and I want to wipe it all so I can restore it from a time machine backup on an external HDD. My understanding is that if I wipe it all (restart, hold command+r, disk utility to erase the HDD), I won't be able to reinstall mountain lion because I never bought it on my Apple ID. What do I do? I heard I can use a flash drive or another HDD (neither of which I have), put the image of mountain lion on that, and install it from that. Is there any other way that doesn't involve me borrowing/buying a big enough USB drive? I have no problem with paying the $20 for mountain lion.