erasing external hard drive question

I removed everything from my external back up and put it all on CDs. After I made sure I had everything I put all the folders in the trash and emptied it ,then rebooted becuse my external still says only 1o GB's free

The external is a Lacie from Apple good for 232GBs. I put all my stuff on CD's to let time machine use the Lacie exclusively. But I can not get it to free up all the space. anyone know how to wipe an harddrive clean?

Use disc utility to erase and reformat the drive.

Thanks for the reply, so disc utility will do it. Do I erase or Restore?

I erased all and it still says only 10 GBs free.. going nuts hrer!