I know everybody says EXT3 and EXT4 filesystems don't need defragmentation and in most cases, that's true! But sometimes they actually do need defragmentation. There is an entire discussion rather or not EXT3 needs defragmentation but I'll not get into that. It just depends on the way a filesystem is used, when it was last formated and how full it is.
I have just discovered that ext3 defaults to reserving 5% of its partition exclusively for root, as a precaution measure that your system does not get FUBAR when you use it for your root partition. I have a 230GB external USB disk that I use for all my big storage requirements, downloaded stuff, backups etc. Due to this reservation I had 11.5GB of unusable disk space, thankfully this is easy to fix: