Turn your old NES cartridge into an external hard drive, just like this guy
did, except with better step-by-step instruction.
I altered the built-in LED so that I could have custom red lights coming out the bottom. This is an optional customization; if you aren't comfortable with cutting some wires and adding your own stuff, don't do it.
NES cartridge (one that you can bear to open up to take away its playability)
External hard drive - a SMALL one (4.5"x2.75" bare, about 5"x3.5" with the case on)
Screwdrivers, of various sizes, philips and flat head
Dremel or other multi-tool
Pencil & paper
2 small LEDs, I chose red to fit with the NES color pallet
Hot glue gun
Helping hands (not your little brother's hands, a stand with clips for holding things while you solder)
Oh yes, and I am so NOT liable if you screw up your hard drive. Perform the operation at your own risk.