Download ((new)) Os X Mountain Lion Bootable Usb -

Restart the Mac and immediately press and hold the key.

If so, you may need to use a virtual machine or older hardware, as newer Macs often block older operating systems. Mac OS X Mountain Lion Installer - Apple Support download os x mountain lion bootable usb

The USB was not formatted as GUID Partition Map. Solution: Repeat Part 5, Step 1. Ensure the Scheme is GUID, not Master Boot Record. Restart the Mac and immediately press and hold the key