Need help with Time Machine exclusion list

[gothamsurfer]

I have a 1TB Western Digital Essential hard drive with about 500Gb of music, movies and other assorted data tethered directly to my iMac. I also have a 1TB Time Capsule that serves as my wireless router and connects to my iMac over the wireless network. I'm trying to use the Time Capsule as the ultimate backup device, to backup the data on my iMac and the tethered 1TB Western Digital hard drive via Time Machine. It already backs up all the data on my iMac, but unfortunately, Time Machine places the 1TB Western Digital hard drive in the exclusion list (which I understand is normal), but will not let me click the minus sign to remove the drive from the exclusion list (which is where I'm at a loss). Someone please help me find a way around this as I've been unable to find a way to remove the Western Digital drive from the exclusion list and have Time Capsule access its data. Many thanks in advance for any help with this.