SD cards briefly talk about their security features and the need to pay the "Secure Digital People" for specs on the device and perhaps even some proprietary information on how to talk to the chip such that it will actually store and retrieve data.
So I suggest you to go for MMC card.
Go to Sandisk's website, and get the MultiMediaCard spec in the OEM
section. Develop your software against the MMC spec. If your
software follows the MMC spec, it will talk to an SD card without
changes. You don't need to know anything about the security features,
and you don't need the SD card spec. The only difference will be that
you'll need an SD-compatible slot, since the SD card is thicker.
**broken link removed** Sandisk SD card product manual
**broken link removed** MiniSD card
**broken link removed** Older SD card
specs
Using SDCard and SDIO with the Intel? PXA250 MMC Controller Application Note
https://www.intel.com/design/pca/applicationsprocessors/applnots/278533-001.htm
Toshiba SD card specification
**broken link removed**