hi, what would be the best way to make a complete osx restore on a 2011 MBA 11" which came preinstalled with osx lion but has now yosemite 10.10.5 installed? The reason to reinstall osx is because i will be giving it to a relative. Can i erase my MBA hd and install 10.10.5 from scratch even though it came preinstalled with 10.7? I already have a usb stick with yosemite 10.10.5 loaded. Thank you!