This a summary of an article describing how to move the root partition found on the Raspberry Pi Forums .
Before you do anything do a “lsusb”without your external storage plugged in. Then plug in your external storage and do a “lsusb” again to make sure that your storage is going to work OK.
Before you start give a bit of thought as to how you want your storage partitioned. I just left my SSD as a single 500GB partition. In the future I can see that I’ll be adding more to house my collection of music and videos.
Anyway, without further ado, if you follow this guide to the letter I’m quite sure you’ll have no problems.