Back up a WD drive to Time Capsule

I have a Mac Mini with a WD VelociRaptor Duo attached. I keep all my software on the Mac Mini but all my work on the WD external drive. Question is how do I tell Time Capsule to include the WD in it’s back ups ?

If I enter Time Machines options it shows the WD drive in it’s _ excluded _ list but it won’t let me click on it to remove it from this excluded list.

Any ideas please.

Thank you John

Is the drive connected to the Time Capsule or to the Mac?