Not being able to remove a drive from Time Machines exclusion list

I am trying to make TimeMachine make a backup of my external drive and think I have followed all the steps needed to do this, plus reading up on older issues people have had, but I am no step closer to a solution.

The TM-drive is a 4 TB Samsung and the drive I want to include is a 2 TB Samsung. It should fit with other words, and it's not even half full either.

I have tried finding hidden .mac files that exclude the drive, and things like that.

I have tried creating a new admin account but it is the same there.

Does the drives filetype matter? I notice that the two drives are different in that regard.

What am I doing wrong?!