Both the command line solution and What's Keeping Me work great.
To recap, command line in terminal is:
sudo lsof | grep /Volumes/myDrive
The GUI application is What's Keeping Me, available~~available at Hamsoft EngineeringHamsoft Engineering]~~ Thomas Templeman1.
Screenshots of both with the same search going on.
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.