But [Neutrino] made his own using an ESP8266 married to a card reader. It all starts with the old trick of soldering a header to an SD card adapter ... Starting from a few examples, he wound ...

SDIO is an extension of the SPI-like protocol that’s used with SD memory cards. The idea with SDIO ... and found that you can liberate the ESP8266’s SPI bus by removing a flash memory chip ...