interesting. It works for me. But erasing content from a disk is easily done using the app "Disc Utility".
However, I am not sure whether your version of OS X has it installed.
If you have it and when you open it, it shows on the left panel all the disks that are available.
Chose your back up disk and go to the tab "Erase". You can even chose from the "security options" how "complete" in terms of recovery the content should be erased. Usually, the default is enough.
Then click on erase and you will find all content gone from the disk you marked on the left panel.
Cheers!
JP