The major reason why an external hard drive won't eject on Mac is that it's currently being used by another app or process. However, this may or may not be your case, as you've got no error notification indicating the drive is used by other programs.
One assumption is that certain bugs on your current system is preventing your drive from being ejected. If a new update is available to your Mac, it's recommended to update your Mac at a proper time. Right now, you can use the following solutions to eject your external hard drive on Mac.
Remember to test if your drive can be ejected after each solution.
- Make sure the app SuperDuper is closed.
- Press Option + Command + Esc, and select Finder > Relaunch.
- Check the processes that are still accessing your drive with the lsof command in Terminal and then quit them.
- Log out of your account and then log back in.
- Shut down your Mac and remove the external drive.
If you're not certain about the steps to execute these solutions, you can find the details in this tutorial: How to Safely/Force Eject External Hard Drive on Mac?