Cannot delete existing files from my external drive

I have a My Book that I purchased about 8 years ago. I wanted to use it to back-up some files, but I cannot delete the existing files that I no longer want. I am using OS X 10.11.1, and the model of the drive is WD6400H1U-00. I hate the idea of discarding the drive when there is nothing wrong with it.

Hello there,

What error are you getting when you try to delete the files?