If you just need a megabyte or two of extra storage for a project, we recommend an Adafruit SPI FLASH Breakout. The breakout is single-channel SPI only, but it comes with level shifting so it can be used safely with 3V or 5V power and logic. You can address them as a flat memory space or, if you like, format them with a filesystem like littleFS or FAT.The Adafruit SPI Flash Breakouts guide has everything you need to get started with using these SPI flash breakouts. There’s pages for overview, pinouts, Arduino and download resources.Check out the full guide: Adafruit SPI Flash Breakouts