I'm about to reinstall the OS on my Macbook via a Genius Bar appointment at a particularly large Apple store in Australia. They have a bootable drive with every OS from 10.4 to the latest one, 10.11, El Capitan. It's been recommended to me that I don't reinstall the OS from an online source, but what's the risk of using the external of the Genius Bar that's presumably been plugged into many Apple products before? BadUSB, etc. Would it be less risker to do it online via the Apple store's unsecured wi-fi? I know for a fact that on a clean install, the Macbook's firewall is not turned on.