What to do when a hard drive won’t unmount in OS X El Capitan

When OS X won't let you unmount a flash drive or external hard drive, here's what you can do....
http://www.macworld.com/article/3061175/os-x/what-to-do-when-a-hard-drive-wont-unmount-in-os-x-el-capitan.html#tk.rss_all