For example, having an Explorer window open should not
>prevent stopping the drive.
ONLY if the cursor focus is NOT on that drive.
That is the main issue.
You cannot be browsing the drive when you intend to stop it.
Your focus has to be moved to another volume.
Then, do NOT right click on the icon, standard LEFT click on it and select the drive you want to unmount. It will with a single mouse click.
The right click method is lame as it requires one to go through two more menus to kill and release the volume.