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.
Mac hard drive icon
Apple

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Glenn Fleishman’s most recent books include Take Control of iOS & iPadOS Privacy and Security, Second Edition (covering iOS 15/iPadOS 15), Take Control of Your M-Series Mac, and Take Control of Securing Your Mac. In his spare time, he makes Tiny Type Museums. He’s a senior contributor to Macworld, where he writes Mac 911.