High capacity SDHC (SD HC) cards currently range from 4GB to 32GB. The capacity of the SD HC card is at present limited to 32GB. However, in principle, it is possible to create SDHC like cards up to 2048GB (two terabytes) capacity.
The SD HC format has proven to be very popular. While allowing capacities greater than 4GB SDHC, the SD HC card has created compatibility issues with older devices that are not able to read this new format, but some devices require only a firmware upgrade to become compatible. SD HC cards require SDHC capable device firmware generally not found with older devices.
File System: SD cards are typically formatted with the FAT16 file system, while SDHC cards are typically formatted as FAT32. However, both types of cards can support other general-purpose file systems, such as UFS2/ext2 or the proprietary exFAT for example.
SDHC cards have SD Speed Class Ratings made by the SD Association. The SD Speed Class Ratings specify the minimum write speeds based on the best fragmented state where no memory unit is occupied, as shown below.
SD HC Class 2: 2 MB/s - 13x
SD HC Class 4: 4 MB/s - 26x
SD HC Class 6: 6 MB/s - 40x
SD HC Class 10: 10MB/s - 65X