iTunes on external Hardrive- Will Time Machine back it up?

I have all my iTunes music and iPhoto pictures on an external harddrive. I have another external Harddrive that I use for Time machine. I want Time Machine to Back up the external harddrive. When I open Time Machine preferences, the list of "Do not backup" contains my external harddrive and when I try to select it and hit the "-" sign it is grayed out. What gives? How can I use time Machine to back up my music and photos? I cant keep all that info on my internal harddrive. Surely Apple must have thought of this. Help!