And so it did. And here's how it happened.
Note: This NES was dead and sold to me as-is. I think the game cartridges still worked, but it's not like you can't play Tetris on everything.
Step 1: Open a Cartridge
Get an old game cartridge and open it up. I used a special bit I got online that's made just for this, but I've heard of others having luck with making their own bits.
Step 2: Modify Cartridge and Add Flash Drive
With the cartridge opened up, use a Dremel to make a hole for the flash drive to stick out.
With the opening, use hot glue to keep the flash drive in place.
Step 3: Open and Mod the NES
Now it's the NES console's turn!
Use a Phillips head screwdriver to take the top off as well as the metal shield inside.
Using a Dremel again, cut a hole in the middle of the 72-pin connector and a small hole in the back for a cord to come out.
Step 4: Inserting the USB Cable
Insert a USB extension cable into the opening in the NES.
Double-check to make sure that the cable is in the right spot and hot glue it in place.
Insert the cartridge to make sure that the flash drive connects cleanly with the USB cable.
If it's good, put the metal shield and plastic top back on and you're done!
Step 5: Enjoy a Ridiculous Item
Presto! You now have a ridiculously inefficient flash drive and a massive USB port. As an added bonus you can make a custom icon for the flash drive so that you know what game cartridge has been inserted.
Step 6: FAQ
Why not a 2.5" HDD instead of a flash drive?
I had a spare 2.5" drive that I was trying out for a while, but wasn't happy with the connection I was getting with it and shelved this idea for a while. When I tried again the drive was dead and didn't want to spend money on a new one just for this. The total spend here was $20 (NES and cable) and I wanted it to stay low. After all, it's not very useful, is it?
Why not use the connector pins instead?
Right, the idea of soldering the 4 USB pads to 4 pins on a game cartridge and something similar to the other side with a cut up USB cable. It would've made the build more impressive, but not change the action. Also, this way it's easy to remove the flash drive and use it as normal.
Why destroy a NES?
Really surprised that so many people care. There are a lot of them around that still work. This one didn't. Yes, I could've put some work into it and make it function again to improve its value by $30 or so, but I'm not interested.
Why not put more memory in there?
You absolutely could. If you want to take this idea further the instructions are here for you.