I'm running Debian (wheezy) on my Raspberry Pi, and I apparently allocated something incorrectly when setting it up awhile back. I have a 16GB SD card, and I'm only using a couple GB for the root partition.
When I plug in the card to my Mac card reader, and this is what I see.

It looks like I have a large chunk of unused space. How do I go about reclaiming this without wiping out my existing data/settings? Looking for command-line instructions.