This will keep your files safe on your external hard drive no matter what happens on your computer. Would you like to answer one of these unanswered questions instead?
To make sure you can keep actually using Parallels, though, you'll need to make sure you're on Parallels version 10 or 11. I know I'm not directly answering your question: on my Book Air i7 8gb ram, after upgrading to El Capitain, my Windows setup (Win XP and Win 10) are running extremely slow from an external usb 3.0 SSD.
Now, with Parallels Desktop 12 including Parallels Toolbox, you can do all of that and more, with one click!
The new update seems pretty legit, so I am considering updating my macbook.
I have many older apps running on a newer OS X and have no intension to update all my apps, only some of them.