Remove external drive from Time Machine
I got an answer from an on-line friend.


  The procedure varies with different versions of macOS but it should be
  something like this:
  
  There's a little gear icon you can have visible in Finder windows. 
  You need this.  So if you have customised the toolbar to remove it,
  put it back in.  You also need the Time Machine icon in your menu bar,
  and to connect your backup drive if it's not already connected.
  
  Open a Finder window and pilot your way to a view which shows the
  volume you want to remove.
  
  Use the Time Machine menu bar icon --> Enter Time Machine
  
  In that Finder Window highlight the volume / files you want to remove.
  
  From the gear icon at the top of that window choose "Delete all
  backups"


I did that and now instead of 294.41GB available on my TimeMachine drive, I have 3.74TB.