1. Physical Interface and Package Form Factor: The MMRD4 uses a standard SD card form factor (approximately 24mm × 32mm), whereas the MURD4 utilizes a microSD card form factor (approximately 11mm × 15mm). Their electrical interfaces and mechanical dimensions are completely incompatible, necessitating different PCB card sockets or connectors. This represents a fundamental difference at the hardware design level. 2. Application Scenarios and Design Suitability: Standard SD card slots are typically used in devices with less stringent space constraints (e.g., industrial cameras, larger embedded systems). In contrast, microSD card slots are employed in highly compact spaces (e.g., drones, miniature modules, ultra-portable devices). A direct substitution would result in a physical form factor mismatch, rendering installation impossible. 3. Potential Minor Differences in Signal and Power Characteristics: While the core memory chip technology and key performance parameters (SLC, Class 6) are identical, differences in the package and interface circuitry may lead to subtle variations in parameters such as high-speed signal integrity and peak current. These aspects require verification through circuit analysis and driver testing.