I somehow lost my MAC partition on my iMac and now I only have Windows 8.1. I want to upgrade to Windows 10 but I want a clean install which I can only get through booting from the usb drive. Whenever I get to the boot menu (after holding the alt key) the usb doesn't appear as an option to boot from. I think this has something to do with the way my usb is formatted because it works fine on other PCs. Remember , I can't use the Boot Camp Assistant which looks to be the solution to this kind of problem. I only have Windows 8.1 on my hands and that's it. Please help me.