If you need to unmount your external hard drive and your drive is not unmounting, then there is usually a application that is not letting the drive be unmounted. In Drive Genius 4, follow the steps below:
Open Drive Genius 4.
Make sure you have the drive backed up.
In Drive Genius 4, choose your hard drive on the left, then go to Active Files and hit start.
If Active Files is successful then you are done.
Quit Drive Genius 4.