You must return it to the original shipping version of OS X. In your case that would be Lion. You can do that from the Online Internet Recovery system. At startup hold down the Command+Option/Alt+r keys until you see a globe on the screen. Once that loads use disk utility to "Re-Partition" the drive as One partition (you must do that to remove the Mountain Lion Recovery HD partition). Once that is done exit Disk Utility and select Reinstall Mac OS X from the list of options.
Lion will be downloaded from the internet (at some point you will need to select your WiFi sign name and type in your password for the WiFi so have them handy) and when the download is finished the system will reboot and the install will start.
ultradadd wrote:
Thanks, LowLuster.
It was Lion; I upgraded to Mountain Lion earlier this year.
How do you suggest I revert back to Lion, without the discs? (MBA)