How to backup one external drive to a second drive using Timemachine

I have data on an external drive which I want to backup to a Timemachine drive. In the Timemachine settings, there is a section called "Exclude from Backups". My source drive is shown in there and although there is a minus sign, it is grayed out, so Timemachine is excluding the drive I want to backup. I tried using iCloud but found out it will not read external drives. How do you guys backup an external drive I have no room on my computer hard drive.