Alan Lees wrote:
I installed Lion successfully on a new external HD - I now want to upgrade that volume to Mountain Lion
Boot from that volume. Go to AppStore purchase ML and DL it. When it finishishes DLing it will open an install window. Before you install, quit out of the installer. Go to Applications and copy "Install OS X Mountain Lion.app" off to some other media. This is because it self-destructs after install, so if you ever need it again you don't have to go through the DL again.
Then double-click the "Install OS X Mountain Lion.app" in your Applications folder and you're off and running.