Time Machine drives won't eject -- always "in use"

I had this problem and none of the solutions I found online worked for me.

BUT THIS DID WORK -- I set-up the Time Machine drive again and this time I told it NOT to encrypt the drive. MacOS warned me that I was backing up an encrypted Mac to an unencrypted Time Machine drive, but I clicked OK to ignore that warning.

Since then, I can eject my time machine external HDD anytime -- even when I see that Spotlight is busy in the background. I hope this helps others too.