Apple even gives you directions on how to do it:<p><a href="https:&#x2F;&#x2F;support.apple.com&#x2F;en-us&#x2F;HT201372" rel="nofollow">https:&#x2F;&#x2F;support.apple.com&#x2F;en-us&#x2F;HT201372</a><p>Save it for however long you&#x27;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.