Comment 6 for bug 670628

Rob, following is some information for how GParted determines the unused space in a partition.

For the ext4 file system GParted uses the following command to retrieve information:

     dumpe2fs -h /path-to-partition

Where /path-to-partition is something like /dev/sda3

Then GParted parses the output to find "Free Blocks:" and "Block size:" to calculate the amount of free space. Please note that free blocks is not necessarily exactly the same as available free space in the file system.

The relevant GParted code can be found in the set_used_sectors method at the following link:
http://git.gnome.org/browse/gparted/tree/src/ext4.cc#n64