SD and SDHC - compatibility
A new SD format, SDHC (Secure Digital High Capacity), allows capacities in excess of 2GB (4GB to 32GB)[5]. SDHC uses the same form factor as SD, but the SD 2.0 standard in SDHC uses a different memory addressing method (sector addressing vs byte addressing[6]).
The following compatibility issues between SD and SDHC cards and devices can arise:
  * Devices that do not specifically support SD2.0/SDHC do not recognize SDHC memory cards.
  * Some manufacturers have produced 4GB SD cards that do not conform to the SD2.0/SDHC and are incompatible with existing SD devices, and are also incompatible with SDHC devices[citation needed].
As of late 2006, the simultaneous availability of non-standard 4GB SD and 4GB SDHC cards, and the incompatibilities between SD and SDHC have caused a great deal of confusion among consumers looking to buy memory for various devices.
4GB SDHC cards can only work with SDHC compatible host devices. The SDHC logo is supposed to ensure compatibility.
SDHC readers and other devices are supposed to be backwards compatible with SD cards.
SDHC cards have SD Speed Class Ratings defined by the SD Association. The SD Speed Class Ratings specify the following minimum sustained write speed for SDHC cards:
  * Class 2: 2 MB/s
  * Class 4: 4 MB/s
  * Class 6: 6 MB/s