Perhaps you've forgotten already how to force eject the external hard disk the last time experiencing the problem. If so, there are several ways you can do to remove it safely without causing problems or damages to the hard disk or hard drive.
Log OutThe first option is to Log Out, i.e click the Apple logo on the top left corner and then click Log Out at the bottom line of the menu. After logging out then loggin in again. Open Finder and try to remove the hard disk.
Shut DownIf it still fails, you can turn off the computer by clicking the Apple logo at the top left corner, then click "Shut Down". Unplug the external hard drive and restart computer.
Before using the above methods you can try first quitting from all programs that are still running. Exit also from Mail, as it might previously have sent an email with attached images or files.
To quit from all the programs in OS X El Capitan, press Command key and press Tab key to view all active applications. Move to an application and close it. Repeat the step until all applications are closed, and try to remove the hard drive.
Occasionally Finder can also be the cause of this problem, for example after using QuickLook to view a file. You can close Finder by using Activity Monitor, or use Quit command using a utility like TinkerTool.
If there are other users using the external hard disk, log out the users and then try to remove the hard disk. If it fails, log in as that user, and try to remove the hard disc.