boot off external HD
I have a new iMac Late 2014 retina 27 inch , it came with 10.2.2
I need Mountain Lion to use iMovie HD 6.0.3.
I have an external HD partition with Mountain Lion installed, and I cannot boot up from the external hard drive in Mountain Lion or 10.9 partition.
My last iMac had no problem booting from the external.
Does anybody have a workaround where I can have this iMac run Mountain Lion?
iMac with Retina 5K display, OS X Yosemite (10.10.2)