I have upgraded my Mac mini 2012 to Mavericks, and everything is running quite well. However, it turns out that I need to also run Mountain Lion sometimes (a piece of hardware I need to use is not Mavericks compatible.)
I am attempting to install Mountain Lion on a 2nd partition on my Mini's internal hard drive by following the following steps:
used Disk Utility to repartition my mini's internal drive, creating a 100 GB partition to install Mountain Lion on
used DiskMakerX to create a Mountain Lion bootable USB key
rebooted machine while holding down Option
At this point the boot menu appears and I see icons for both my Mavericks partition, as well as the Mountain Lion installer USB key. However when I select the USB key and try to boot from it, it does not boot; instead I get a gray screen with the universal "no" symbol (circle with a slash through it) on it.
I tested the USB key on another Mac and it booted fine, so there is nothing wrong with the USB key I made. So it would seem that the Mountain Lion installer will refuse to run on a computer if it has a newer OS on it, even if the computer has a free partition (or drive) to install on to.
Is there any way around this restriction?