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:
tune2fs -m 0 /dev/sdf1
Replace sdf1 with your partition name. You don’t even have to unmount your disk. Voilá, 11.5 GB more space for free Here is the output of df -h as proof:
Filesystem Size Used Avail Use% Mounted on /dev/sdf1 230G 193G 26G 89% /media/disk
Filesystem Size Used Avail Use% Mounted on /dev/sdf1 230G 193G 38G 84% /media/disk
If you like this, you might also be interested in How to change Ubuntu forced fsck.
Update: The free space limitation is also used to prevent fragmentation. So if you set the limit to zero and operate on a very full harddisk for a while, your filesystem might slow down.