Not impossible at all, I did it very recently myself, its very easy, here's a set of instructions on how to do it:
http://lifehacker.com/how-to-move-a-pc-game-to-another-hard-drive-without-re-1714706774
I used option 2 in those instructions by the way, lets call your original drive C and your new drive D.
1. Create a folder in the new drive where you will store your games. If you’re on Steam, you’ll need to do it through Steam. Head to the top menu's select Steam > Settings > Downloads > Steam Library Folders and click “Add Library Folder” call it Steam
2. Navigate to your new Steam library folder on D drive, and create a new folder within it called steamapps. Then, create a folder in steamapps called common.
3. Head to your current Steam folder on C and find the folder for the game you want to move. You’ll likely find it in C:\Program files (x86)\ steam\steamapps\common. Copy the game’s folder, e.g. “Borderlands 2”, to the new D:/steam/steamapps/common folder you created in step 2.
4. Open Steam app, right-click on the game you've moved, and select “Delete Local Content”. This will uninstall the game from its original location.
5. When that’s done, click the Install button to re-install the game. In the Choose D:\steam\steampps\common as the location
Instead of re-downloading the game (which could take hours), Steam will detect the existing files there and make any necessary minor changes. When it’s done, you should be able to play the game as usual.