The Mega Man NES cartridge you see here is actually a...
...500 GB hard drive. That's according to a Russian Doll's worth of links that point to a company called seller on tech crafts site Etsy which sells hard drives that masquerade as NES cartridges. And they won't just do fake Mega Man cartridges for your $149. Any game's possible.
Mega Man Custom Hard Drive Enclosure - 500 GB USB [Via Capcom Community Blog]