From
http://en.wikipedia.org/wiki/Secure_Digital :
Compatibility with SDHC
SDXC host devices accept all previous families of SD memory cards.[26] Conversely, SDHC host devices accept SDXC cards that follow Version 3.0, since the interface is identical,[3] but the following issues may affect usability:
SDXC cards are pre-formatted with Microsoft's proprietary and patented exFAT file system, which the host device might not support. Since Microsoft does not publish the specifications of exFAT and its use requires a non-free license, many alternative or older operating systems do not support exFAT for technical or legal reasons. The use of exFAT on some SDXC cards may render SDXC unsuitable as a universal exchange medium, as an SDXC card that uses exFAT would not be usable in all host devices. Since the FAT32 file system supports volumes up to the SDXC's maximum theoretical capacity of 2 TB as well, a user could reformat an SDXC card to use FAT32 for greater portability between computers (see below). FAT32-formatted SDXC cards can be used in a host device built for SDHC if the host device can handle 64 GB and larger volumes.
SDHC host devices do not test the new capability bits defined for SDXC 4.0 cards and thus cannot use the new features of SDXC, such as transfer speeds above UHS104 (104 MB/s).
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