How do I tell itunes to use a different folder on a different hard drive?

Go to Edit>Preferences> Advanced tab to see where your library currently is
Is "Copy files to iTunes Media folder when adding to library" ticked?

If yes

Go to the folder location of your current library, move it to your new drive and then in iTunes go back to Edit>Preferences> Advanced tab and repoint at your new directory

http://support.apple.com/kb/HT1364
http://support.apple.com/kb/HT1203
http://docs.info.apple.com/article.html?path=iTunesWin/9.0/en/27554.html

This is the best guide:
http://www.ilounge.com/index.php/articles/comments/moving-your-itunes-library-to -a-new-hard-drive

Do NOT move the _iTunes media folder_ yourself or you will lose everything.

iTunes prefs -> Advanced.
Tick *Copy files to iTunes music folder when adding to library*.
Tick *Keep iTunes music folder organized*.
Set the *iTunes Media folder* to the location you want it to go to.
Then File -> Libary > Organize library > Consolidate.
This will copy everything in iTunes to the new location and use that for everything from now on.

Ah yes - that is very true - apologies!

Sometimes moving the iTunes media folder yourself may work but iTunes is not designed that way and more than likely it will lose some or all the music so I never suggest doing it that way.