If you need to move a game from one harddrive to another this is extremely easy to do.
- Decide where you want the files to be stored this needs to be done through steam as follows:
Settings -> Downloads -> Steam Library Folders
- click Add Library Folder
- Navigate to your new steam folder and create a folder in steamapps called common
- Head to your current steam folder and find the game you want to move.
You will probably find it in:
Copy the game's folder to the new folder you created in step 3
- Open steam, right click on the game your moving and select
Delete Local Content
- When that is done clock the Install button to re-install the game, in the dropdown choose the steam folder you created in step 3
- Instead of downloading the game steam will detect the existing files and make any minor changes.
When this is done you should be able to play the game as normal.