Modders are really just geekier versions of Tim Allen. Take Natrium42 - if he weren't adding 40gig drives to Nintendo DS units, he'd probably be fitting dragster engines into snow blowers.
Essentially, Natrium exploits the IDE interface of CompactFlash for the hack, which was cool enough to pull me out of retirement for at least one more post. His step-by-step is very clearly assembled for anyone interested in giving up a lot of their time. My favorite part was Natrium's melancholy acceptance of success: "And that's the end of this hack. Now I need to find something useful to do with my 40GB NDS system."