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.
Step 1: Open Her Up
I've heard that melting the tip of an appropriately-sized plastic thing with a lighter and sticking it in the hole to let it cool will result in a little tool that you can use to unscrew the custom screws. I'm not responsible if you end up melting a Knex piece to your hand.
Anyways, unscrew the thing, whichever way you need to do it.
And remember to save ALL of the screws!
Open the cartridge and take out the little circuit board. Smaller than you thought it would be, huh? Keep the cartridge; you might use it in some wacky art project some day.