Apple even gives you directions on how to do it: https://support.apple.com/en-us/HT201372 https://support.apple.com/en-us/HT201372 Save it for however long yo

Apple even gives you directions on how to do it: https://support.apple.com/en-us/HT201372 https://support.apple.com/en-us/HT201372 Save it for however long you'd like. I actually used my Mountain Lion thumb drive to boot my Mac to fix the problem with my Yosemite thumb drive when it failed to install a bootable Yosemite.