So I was about to re-partition my 500GB external HDD to be a bootable drive for Ubuntu 12.10. But when I started up GParted it showed that the only partition has used ~100GB.

But in reality, I know that I have only used ~30GB, and both the Disk Usage Analyzer and the Properties page for the mount point confirm this.

Why would GParted show that I have used so much more space than I actually have? Is it just space that deleted files may be taking up, or is there something I should be alarmed about?
Thank you in advance.