Maybe it's my unfamiliarity with how 4-bit SDIO works, but I'm curious as to how the SS pin for an SD card can be replaced with BUILTIN_SDCARD on the Teensy 3.5/6. Does SDIO also use a slave-select mechanism? The wikipedia page is unclear as to what the transfer protocol differences are between SPI, 1-wire SDIO and 4-wire SDIO.