Template:FAQ/Questions/sd cards interchangeable with

Are SD cards interchangeable with MMC cards?
No, they are not interchangeable — the cards are physically the same size and have the same connector, but SD uses an undocumented encryption scheme to communicate with the controller, and for the controller to communicate with the outside world. The SD cards are not supported in u-boot.

Actually, it's not undocumented, but the fee is in the range of $5000 to get the docs, and also requires a licensing agreement.

As such, only use MMC cards, not SD cards.