I’ll skip the obvious stuff about having to own a Raspberry Pi and external hard drive and how to set up the Pi since the first two should be obvious and the third you can find 1,000 other places. Instead I’ll just cut to the chase. This worked for me on a Raspberry Pi 2 and so it should work on a 3 as well.
I found the instructions here. They are mostly right but the formatting is a little inconsistent and I thought some of the steps were out of order so I’m going to summarize it, clean it up, and inject what I learned from my experience. Instructions after the fold.