Time Machine won't recognize new external hard drive

I just installed a new external hard drive to my G5. My primary (internal) hard drive is currently being backed up by Time Machine to a second internal hard drive. A third hard drive contains all my photos and graphic art works. I purchased the new external drive to back up the third drive (art works). Even though the new drive appears on my desktop, it is not recognized in Time Machine as a drive I can use to back up files. If I select the "Options" button in Time Machine, the new drive shows up on that menu as one that can be excluded from being backed up. How do I get Time Machine to also back up files from my third drive to the new external hard drive?