1. Physical Form Factor & Interface: This is the fundamental difference. The MMRD4 is a standard SDHC card, whereas the MURD4 is a microSD card. They utilize entirely different physical connectors and package dimensions. If the host device features an SD card slot, it cannot physically accommodate a microSD card. While a physical adapter can enable electrical connectivity, it introduces additional mechanical contact points. This may compromise long-term reliability in scenarios involving vibration, extreme temperatures, or frequent mating cycles. 2. Series & Design Target: The differing series codes (MMRD4 vs. MURD4) indicate these are distinct product designs tailored for different form factors. This typically implies differences in internal PCB layout, component stacking, and potentially firmware or power management optimized for the respective size—even if the core NAND flash and controller technology share a common origin. 3. Price: The MURD4 (microSD) carries a significantly higher price. This generally reflects the greater technical challenge of achieving equivalent capacity and reliability within a smaller package (e.g., higher-density integration, thermal management, signal integrity preservation), leading to increased manufacturing costs. The price differential is the direct commercial manifestation of this physical disparity.