You can move individual games easily:
1) Close steam
2) Make a folder for your steam game files on the new harddrive
2) Go to your steam folder, then steamapps, then common.
3) Find the game you want to move, cut and paste the folder into the new steam folder on your new drive.
4) Open a command prompt and type the following command:
mklink /J "original directory" "new directory"
For example, my steam is installed to my C drive and I move games which dont require the benefit of my SSD to my mass storage E drive.
mklink /J "C:\Program Files (x86)\Steam\steamapps\common\torchlight" "E:\Games\Steam\steamapps\common\torchlight"
5) Open steam, and ensure the game still works.
This works by creating a symbolic junction between the two folders, windows (and steam) see the new folder as if it were located in the original directory.
I personally just put together a quick batch script which does the above automatically when i decide I want to move a game off my SSD.