Only 11 years late to the game… with ESPs now ubiquitous (at 3.3V, so no level shifter needed), I ended up using SPI mode to get the raw image of an otherwise unaccessible SD card (and evtl ...

– May be a card for programming the firmware on your AVR project since you already bring the SPI pins to the SD connector – just need the reset line. That might actually be cool. – A gag item.