Here's all you do.
- First, tell iTunes to start storing music on a new drive (Preferences > General and clicking the Change button under 'iTunes Music folder location'). All music you import from now on will go to the new folder location ... but iTunes will continue to recognize and list all of your music from your old folder as well. Indeed, you won't know what songs are in your new folder and which are in your old folder. It's seamless.
- Now let's move some of your music over to the new folder. Go to your old iTunes drive (usually [home] > Library > Music > iTunes Music Folder). Open the folder and you'll see more folders, labeled by artist.
- Select some artist folders from the old folder location and drag them into iTunes. I drag them over the iTunes logo in my dock, but you can also drag them over the iTunes window.
If you're like me, you'll want to test this out before deleting your old music folders. Highlight a song from an artist whose folder you moved and select Show in Finder from the pop-up menu. A Finder window should open with the song file located deep inside your new iTunes Music folder. (Or, just open your new iTunes Music folder and navigate until you see the copies of the folders you dragged into iTunes.)
When you're satisfied, you can start moving your iTunes folders off your hard drive and free up some valuable gigs!

