Downgrading to Mountain Lion on 2012 Mini

I need to downgrade to ML on a Mini that came with Mavericks.

Has anyone done this successfully?

I made a bootable ML USB flash drive.

When I boot off if it, it won't let me install to the Mac HD.

However, it seem like it will let me install ML on the second drive(I have the server Mini w/two HDDs).

This is all a test right now, as I'm replacing both HDDs with an SSD for the boot drive and a 7200 HDD for data.

I had planned on installing ML on the original HDD and clone it to the SSD and mount this externally to test it before I open up the Mini.

Any ideas?