It's proper for the operating system to not unmount a volume if there is, on the volume, an open file that can not be closed.

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The accepted answeraccepted answer to The volume can't be ejected because it's currently in useThe volume can't be ejected because it's currently in use

• note the comment about use of sudo

The Mac App Store page for WhatsOpen

Posts that mention What's Keeping Me?, e.g. http://apple.stackexchange.com/a/22268/8546https://apple.stackexchange.com/a/22268/8546

There's another comparable app but at the moment I can't recall its name.