Currently adding existing Steam library folders and/or data to a new Steam installation is a less than efficient process. If you would like to know how to fix new Steam library folder must be empty errors with your new installation read on.
Each new Steam library folder when added through the GUI must be empty at first. With that in mind you need to,
First Create new folders>then add this folder to Steam>then close Steam>then add the pre-existing data into the folders manually>then Open Steam>and finally have access your games.
Urgh, not the best process when you consider how much pre-existing data you may have already pulled down during you last Steam install. It is of course better than nothing and for some better than re-downloading all of your content. Thankfully there is a better way, the manual way; as with all good things you will get your hands dirty, although not too dirty. Included below is how to fix new Steam library folder must be empty error. Both the GUI way and the Manual way are present.