[Flash Card Basic] What is the difference between UHS-I, UHS-II, and UHS-III interfaces?

These refer to the interface bus speed, which dictates the theoretical maximum data transfer speed between the card and the host device.

• UHS-I (Ultra High Speed - I):
  • Max Theoretical Speed: Up to 104 MB/s.
  • Physical: Uses a single row of pins.
  • Compatibility: Most common standard for many cameras, drones, and smartphones.
• UHS-II (Ultra High Speed - II):
  • Max Theoretical Speed: Up to 312 MB/s.
  • Physical: Features a second row of pins on the back of the card for increased bandwidth.
  • Compatibility: Requires a UHS-II compatible device and card reader to achieve these speeds. If a UHS-II card is used in a UHS-I device, it will fall back to UHS-I speeds. Often used in high-end DSLRs, mirrorless cameras, and professional camcorders.
• UHS-III (Ultra High Speed - III):
  • Max Theoretical Speed: Up to 624 MB/s.
  • Physical: Also uses the second row of pins, but with an even faster signaling interface.
  • Compatibility: Less common currently, designed for future high-bandwidth applications.
• SD Express (PCIe Gen3 NVMe): This is the newest and fastest standard, utilizing the PCIe Gen3 NVMe interface (similar to M.2 NVMe SSDs).
  • Max Theoretical Speed: Up to 985 MB/s.
  • Physical: Retains the SD card form factor with a second row of pins, but uses different electrical signals.
  • Compatibility: Requires specific SD Express host devices and readers. It is backward compatible with UHS-I devices (operating at UHS-I speeds). SanDisk has introduced microSD Express cards.

Choosing the Right Card: Always match the card's interface (UHS-I, UHS-II, SD Express) to what your device supports. A faster card in a slower device won't yield the higher speeds and might be an unnecessary expense.