The volume can’t be ejected because it’s currently in use

Disk Utility is in the Utilities folder of the Applications folder. Launch it. IN the left window pane select the hard drive name, not the device. (See photo below for illustration). Run Disk First Aid.

Device name is Toshiba - don't select that. The disk name is TM. That's what you want to select.

When an application has accessed a file that file is flagged as open. If a drive has any open files you'll get this message if you try to eject it. So there are two possibilities: 1) an application really has accessed this file or 2) the file is still flagged as open for some reason. You'll sometimes see this when an applications crashes. Ideally you don't want to force the drive to unmount because the file in question might become damaged or lose changes that you made to it.

When I see this error I first quit all my applications - the ones that would have open files - like the word process, paint program, etc. If that doesn't do it I'll generally restart the computer. That almost always solves the problem but if it doesn't - or if the restart is halted by the Finder then yes, I'll just force unmount. If I do force the unmount I'll always run Disk Utility on the drive.