Both the command line solution and What's Keeping Me work great.

To recap, command line in terminal is:

    lsof | grep /Volumes/myDrive

The GUI application is What's Keeping Me, available at [Hamsoft Engineering][1].

Screenshots of both with the same search going on.

[![Command line output][2]][2]
[![GUI for What's Keeping Me][3]][3]

I prefer the command line just slightly, as it's always there with you. On the other hand, What's Keeping Me is quicker and easier to use if you've got it installed.

  [1]: http://www.hamsoftengineering.com/products/wkm/wkm.html
  [2]: https://i.sstatic.net/yfWrl.png
  [3]: https://i.sstatic.net/4oQBZ.png