Backing up external HDD on Time Machine

[capitanbuzo]

Here is the problem. I have been trying to get Time Machine to back up one of my external drives. When I go into Time Machine's preferences and click options to see the ignore list, the drive I wish to backup is there and the - sign is solid, not allowing me to select it. I have made sure it is highlighted when I try to remove it from the list but it doesn't want to work for me. I know that the Time Machine drive has the space to back this drive up but for some reason it will not let me do it. All help is appreciated.