I'm getting an error code -8003 when I try to empty my trash.

I recently emptied several old backup files from an external hard drive to the trash folder. Now when I attempt to empty the trash, I get this message: "The operation can't be completed because an unexpected error occurred (error code -8003). What should I do? I just took the computer and external hard drive to an apple store for genius support a few days ago and the tech fixed a few things. He advised me to empty the backup files, which is what I'm trying to do.