Hi there, A few days ago I purchased and downloaded mountain lion. I intended to perform a clean install from a usb drive, but didn't have one to hand. Knowing I'd have to wait untill today to get hold of one, I decided to install ML normally in order to check it out before doing a clean install. I think this may have been a mistake. I have now put ML onto a usb drive successfully (it is recognised when I plug it in inside ML) but when I try and restart my macbook in order boot from it, it isn't recognised. I only have the option of booting from my local hdd or from ML's recovery partition. Perhaps this has something to do with ML already being installed, and therefore it see's no reason to install it again? This is pure conjecture, I'm certainly not that knowledgable. Any advice would be appreciated, I really would like to do a clean install. Thanks!