I had been doing some searching around a while back because I had a problem with Steam. Once you install it to a hard drive, ALL games must go there as well. I have a 60 GB (64 GB) solid state disk, and that is just not enough space for all of my steam games. After much searching, I found this:
http://stefanjones.ca/steam/
With SteamTool, you can create an empty folder and name it whatever you want, and then pick and choose what you want to transfer over. It's very easy to figure out and to do. It frees up space and is, as I just learned, a great way to back up your games! I just reformatted and used SteamTool. I had to create a new folder for it and pick one game to install, but then I copied over all the other games from my previous SteamTool folder and Linked them to my Steam. I copied my whole Steam folder from my C drive before the reformat and then replaced it carefully (manually) to finish things up. Make sure Steam is not running for that last part. I tried L4D2 first, and it did not work until I had it verify local content (in steam, right click on the game, go to properties, click on the local files tab, and then click "Verify integrity of game cache". It replaced 2 files and I was on my way!

lol... what a turd.