I think I have done this before, but before I tried again I wanted to check with others to make sure that I was correct. What I need to do is upgrade a lion system to Mountain lion, but when I try to run the installer from within Mac OS, it goes nowhere because of some corruption issues I think with the installer within Mac OS. What I think I can do is boot the computer with the option key to my bootable Mac OS USB stick and install from there. What I want to verify is that I can do an upgrade in that way, not just a clean install. Any confirmation that that's possible?