An adaptable-capacity Secure Digital (SD) card operates as a standard-capacity SD card for a standard-capacity SD 2.0 or 1.x host, and operates as a high-capacity SD card when connected to a high-capacity SD 2.0 host. A 32-bit argument received in a SD bus transaction from the host may be a 32-bit address, which can access 4 G bytes of flash memory in standard-capacity mode. For high-capacity mode, the addressable unit is a 512-byte sector, greatly increasing the addressable memory size. A SD protocol interface on a controller chip performs handshaking with the host to determine the SD version and memory capacity of the host. Host addresses are sent as byte or sector addresses to a flash memory manager on the controller chip, depending on the capacity mode agreed on during the handshaking. Memory areas on the adaptable-capacity SD card for high and standard modes can be separate or overlapping.