Can't install Mountain Lion on a machine that has Mavericks installed, even on a separate disk/partition 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?
The only thing you can do is install ML on an external drive on another mac running 10.8.x then copy the partition using SuperDuper on your 10.9 mac

Let's say your mac is A, 10.8 mac is B and usb HDD is C :


Install Mac Os 10.8 on C using B computer
Create a partition on A (with ± 50GB)
Duplicate C on the A new partition using SuperDuper


And voilà !