Time Machine backing up external devices

I have a 1 TB iMac and noticed that Time Machine was running large back ups and so I looked and noticed that the two external drives I have temporarily hooked up to the iMac weren't excluded, so from what I've read it looks as though those drives were backed up. I now have set these to be excluded. In any case, keeping these drives backed up seems to be taking quite a bit of storage space away from my WD My Cloud. Is there anyway to have Time Machine "forget" these external drive backups and, if not, do I need to start an entirely new backup cycle. How do I do that?

Thanks so much,

Matt