Wait, external hard drive? Yes, external hard drive.
Etsy hardware creator 8BitMemory has a bunch of cartridges reworked into external hard drives, this is just the prettiest. It's also the most elusive, because it doesn't seem to be available in the the store anymore.
You can, however, enjoy an NES controller USB hub, or maybe a Teenage Mutant Ninja Turtles 1TB hard drive. There are a ton of other classic game options, as well, and most can be made to order and suit your storage needs from 500GB to 1TB.