Eyefi cards 4gb and larger are all SDHC format cards. Because SDHC works differently than standard SD cards, this new format is not backwards-compatible with legacy SD format cameras. SDHC 4GB and above cards can only be read by SDHC cameras. Please refer to your camera owner's manual or your camera manufacturer's website to check to see if your camera supports SDHC memory cards.
NOTE: The SD High Capacity (SDHC) card is an SD memory card based on the SDA 2.0 specification. This new SDA 2.0 specification enables SD cards to reach higher capacities: 4GB and higher. For more information, please refer to the Wikipedia Secure Digital card topic for more information.
To see if your camera is compatible with Eyefi cards, please check your camera's owner's manual.