One Mac, two versions of OS X

Reader Will Dawes would like to have a foot in two worlds. He writes:

I have an iMac that’s a couple of years old on which I run Mac OS X Snow Leopard. I’ve stuck with Snow Leopard because I need to run a couple of old PowerPC applications on it that don’t work under Lion or Mountain Lion. But now I find myself in a position where I also need to run Mountain Lion for a job I’m working on. Is there a way to run each on a single Mac?

Yes, at least two ways. The first is to purchase an external hard drive, format it for your Mac, and install Mountain Lion on it. When you need to swap between operating systems, simply launch System Preferences, select the Startup Disk preference, and choose the disk that contains the operating system you need.

Of course, this requires that this Mac is compatible with Mountain Lion. Mountain Lion demands one o these Macs:

• iMac (Mid 2007 or newer)
• MacBook (Late 2008 Aluminum, or Early 2009 or newer)
• MacBook Pro (Mid/Late 2007 or newer)
• MacBook Air (Late 2008 or newer)
• Mac mini (Early 2009 or newer)
• Mac Pro (Early 2008 or newer)
• Xserve (Early 2009)

as well as OS X Snow Leopard version10.6.8 or OS X Lion already installed, 2 GB or more of memory, and 8 GB or more of available space.

The other option is to run Mountain Lion in a virtual environment using a tool such as Parallels’ $80 Parallels Desktop for Mac or VMware’s $50 VMware Fusion 5. Each of these products can run Lion or Mountain Lion (and a host of other operating systems) in a virtual environment. Regrettably the Snow Leopard license doesn’t allow that version of Mac OS X to be run in a virtual environment (though you can run Snow Leopard Server under virtualization). So if you have a new Mac that won't run Snow Leopard, you're largely out of luck.