Why can't I remove external hard drives from the "Exclude from Backup" list in Time Machine?

I have 3 external hard drives connected to my M2 Mac Mini. I have one drive set up as my Time Machine backup. When I click "Options" to remove the other 2 external hard drives from the exclusion list, the minus icon is greyed out. I really want to be able to back those drives up to my Time Machine drive. I've searched for a solution for this, but none of the recommendations I found worked for me.