no public documentation nor any documented legacy implementation is available. All SD cards can be accessed freely using the well-documented SPI bus. xD cards are simply 18-pin NAND flash chips in a special package and support the standard command set for raw NAND flash access. Although the raw hardware interface to xD cards is well understood, the layout of its memory contents—necessary for interoperability with xD card readers and digital cameras—is totally undocumented. The consortium that licenses xD cards has not released any technical information to the public. Data recovery A malfunctioning SD card can be repaired using specialized equipment, as long as the middle part, containing the flash storage, is not physically damaged. The controller can in this way be circumvented. This might be harder or even impossible in the case of monolithic card, where the controller resides on the same physical die. See also Comparison of memory cards Flash memory Microdrive Serial Peripheral Interface Bus (SPI) Universal Flash Storage References External links SD Association Official Site SD simplified specifications SD Memory Card Formatter How to Use MMC/SDC elm-chan.org, December 26, 2019 Optimizing Linux with cheap flash drives lwn.net Flash memory card: design, and List of cards and their characteristics linaro Independent SD Card Speed Tests Types of Memory Cards and Sizes Computer-related introductions in 1999 Computer storage devices Japanese inventions Solid-state computer storage media