An extended Secure-Digital (SD) card has a second interface that uses some of the SD-interface lines. The SD card's mechanical and electrical card-interface is used, but 2 or 4 signals in the SD interface are multiplexed for use by the second interface. The second interface can have a single differential pair of serial-data lines to perform Universal-Serial-Bus (USB) transfers, or two pairs of differential data lines for Serial-Advanced-Technology-Attachment (SATA), Peripheral Component Interconnect Express (PCIE), or IEEE 1394 transfers. A card-detection routine on a host can initially use the SD interface to detect extended capabilities and command the card to switch to using the second interface. The extended SD card can communicate with legacy SD hosts using just the SD interface, and extended SD hosts can read legacy SD cards using just the SD interface, or extended SD cards using the second interface. MultiMediaCard and Memory Stick are alternatives.