As if war, famine and inequality weren't enough to bring us down, Mac users also have to live with the knowledge that some poor souls have no choice but to crazy up their hard drives with secondary operating systems. Where Windows is concerned, many Mac users opt to use OS X's free Boot Camp partitioning software to make their Mac a lean, mean dual-booting machine. Unfortunately, doing so means you'll be losing a significant amount of the hard drive space that was once available to your Mac. A less hard disk hungry method for getting a secondary OS on to your Mac is to install it into a virtual machine, commonly known as a VM.  By using a virtual machine application such as Parallels, Mac users can run set up as many VMs running any number of different operating systems on their computer as they want, and all from the comfort of OS X.

But what if your primary hard drive is so full of Mac applications that you use on a regular basis that there's no space left over to shoehorn in a virtual machine? You could choose to delete a few of those applications and reinstall them on an as-needed basis, or if you have an external drive available, you can show you new virtual machine who's boss by installing it there. We're sure you'll agree that the latter option is a lot more desirable. If you've got a copy of Parallels and a spare USB drive kicking around, here's how it's done.

Step One: Install Parallels

While any of the virtual machines you want to run can reside on your external drive, the Parallels software has to be installed to your computer's primary hard drive. If you're cramped for space, don't panic: for most versions of the application, the installation will take up under 400 MB. Follow the instructions laid out for you by the software's installer. It's a quick and reletively painless process, taking no more than five minutes to complete. Depending on the version of Parallels you use (for this how-to, we're rocking Parallels 5), you may be prompted to update your software. Click yes and move along to Step Two.

Step Two: Set Up A Virtual Machine

With Parallels installed, it's time to get down to the nitty-gritty of setting up your virtual machine. If this is the first virtual machine you've set up, Parallels will automatically ask you what you'd like to install. If you've implemented other VMs on your Mac in the past, click on Parallels' File menu located in OS X's top menu bar and select new. If you plan on installing your virtual machine's operating system from a CD or DVD, be sure that the disc is in your computer's drive before clicking next. In most cases, the software will autodetect what operating system is present on the disc and walk you through the appropriate steps to install it.

If you're installing you VM's operating system from a disc image, you'll need to take the time to show Parallels where the disc image is located before proceeding. To do so, click the Skip Detection button. Parallels will ask you a few pointed questions as to how you want your new virtual machine configured. Once you've answered them, it's time to tell the application where you want your new virtual machine to reside. Remember that external USB drive we mentioned at the start of the tutorial? Make sure it's plugged into your Mac, and move on to Step Three.

Step Three: Tell It Where To Go

No matter whether Parallels automatically detected your new operating system or if you had specifiy where to get it and how to install it by default, the application will want to install your new virtual machine to your computer's Document folder. We don't want that. To make Parallels install the virtual machine on to your external drive, click the Location drop down menu and select the name of your external drive as the destination. Once that's done, you can click the Create button and kick back and let Parallels work its magic.

Step Four: Enjoy!

With the virtual machine installed to your external hard drive, you're ready to reap the benefits of having a secondary OS at your disposal without having to sacrifice any of your Mac's precious primary hard drive space. Remember, you can repeat this process for as many virtual machines as your external hard drive can hold.

Happy computing!

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