Well, sort of. I installed Snow Leopard in one my HDs (mechanical WD Raptor at 10,000rpm). - I have Yosemite installed in a SSD. I have to say, Snow Leopard is MUCH FASTER and more stable than this Yosemite they came up with. It boots faster (even not being in a SSD) and the softwares I use (Logic Pro and Final Cut) run faster as well. My machine is a 6 core at 3.46GHz, mid 2010 5,1 version with 32GB RAM. I'm staying with Snow Leopard for as long as I can -- what a great OS! Has anyone here downgraded their Mac Pros as well? I had to chase down the gray DVDs version 10.6.4 to be able to install, the 10.6.3 that apple sells won't install in my machine. After 10.6.4 installs, upgrading to 10.6.8 is a snap.