I have a 500 GB LaCie external hard drive showing that 17.43 GB available. What should I do?

I have a 500 GB LaCie external hard drive showing that 17.43 GB available. Should I be concerned and purchase another external hard drive? I am confused by a statement I read that indicated when an external hard drive is nearing full, older files will be deleted to make room for new backups. (Or something to that effect). My OS is 10.6.8.