Have you ever tried to format a disk in Mac OS X and Disk Utility won't Unmount the Hard Drive for formatting? It can happen for external (USB and Firewire) as well as internal disks.
In this video you will learn how to force a disk to unmount when Disk Utility won't do it.
You will need a Mac running OS X and this is supported in all versions right up to Mac OS X 10.12 Sierra. Here, we are demonstrating wit Mac OS X 10.10 Yosemite.

The commands you will need are:
diskutil list
diskutil unmountDisk force /Volumes/VOLUMENAME
diskutil unmountDisk force /dev/diskN

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