A crazy guide was posted over at Uncyclopedia which shows you how you can add a normal 200GB HDD to your Apple iPod nano. The solution isn't really elegant nor useful but it is cool..
Now you have 200gb of storage space, fill it up with songs! We found in our benchmarking results that the addition of the ATA hard drive adversely affected battery life. However, we believe that this is a small price to pay for an over 50x increase in storage capacity. Using Apple's own bitrate calculations, we can now store over 150,000 minutes of music! You'll never listen to the same song twice. This is such a great advantage over the original model that we urge all iPod Nano owners to upgrade immediately.
However, a HDD is quite big and there's another major drawback. While the retail iPod nano featured a battery life of 8 hours and 23 minutes the one upgraded to 200GB only features a 6 minute battery life! Check out the full guide over here.