Anyone good with external hard drives?

I have a WD “My Passport” external hard drive. I have a Mac. I use a backup program, “Super Duper,” that does not recommend unplugging the hard drive during a backup. Well, I had to, and now it’s screwed up.

It says there is no room left, but I know there is. Thus, it won’t let me put anything on it. What can I do to fix it? Is all hope lost?