I have a Macbook Air (13-inch, Mid 2011) running OS X Yosemite. I recently installed Parallels 10 and Windows 7 on my internal drive. My mac starts working harder (the fan begins to spin faster) when I open windows, even to do the smallest of tasks. Is there anyway I can reinstall windows 7 on an external hard drive and only run windows when plugged in? Would that even help to keep my mac running efficiently?