From Secure Digital card on Wikipedia: (The "condensed version" is highlighted.)

--------------------------------------------------------------------------------

Compatibility with 2 gigabyte and larger SD cards has been poor, due to the SD/MMC protocol's using a 32-bit address field denominated in bytes. The SDHC standard addresses this limitation by using 32-bit block addresses instead. Both SD and SDHC are traditionally accessed as 512-byte blocks on 512-byte boundaries, so the change to host software or firmware is minor but required. Before SDHC was standardized, various manufacturers "extended" the SD control block fields for their 2 GB and 4 GB cards in different ways. Those cards are incompatible with many SD and some SDHC devices, as they conform to neither standard. All SDHC readers work with standard SD cards.

Many older devices will not accept the 2 GB size even though it is in the revised standard. The following statement is from the SD association specification:

"To make 2 GByte card, the Maximum Block Length (READ_BL_LEN=WRITE_BL_LEN) shall be set to 1024 bytes. However, the Block Length, set by CMD16, shall be up to 512 bytes to keep consistency with 512 bytes Maximum Block Length cards (Less than and equal 2 Gbyte cards)."

Since all cards up to and including the 1 GB card use a fixed 512 block size, some device drivers do not handle the larger block size and will not even recognize the 2 GB card. For example, the SanDisk web site shows examples of devices such as the iPAQ 1910 that will support only 1 GB cards and the Epson Photo RX300 Technical support says they support only 1 GB cards in the SD slot. Users of many early card readers have found that they also support only cards up to 1 GB.

--------------------------------------------------------------------------------

An extension of the SD standard, SDHC (Secure Digital High Capacity, SD 2.0), allows capacities in excess of 2 GB. SDHC cards are often formatted with the FAT32 file system, which supports partition sizes greater than 2 GB. It 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), thus theoretically reaching a maximum capacity of up to 2048GB. SDHC cards only work in SDHC compatible devices, but standard SD cards work in both SD and SDHC devices. The SDHC trademark is licensed to ensure compatibility.

--------------------------------------------------------------------------------

As of early 2007, the simultaneous availability of non-standard 4 GB SD and of standards-compliant 4 GB SDHC cards, and incompatibilities between SD and SDHC have caused confusion among consumers buying memory devices.

SD and SDHC cards and devices have these compatibility issues :

* Devices that do not specifically support SDHC do not recognize SDHC memory cards.
* Some manufacturers have produced 4 GB SD cards that conform to neither the SD2.0/SDHC spec nor existing SD devices.
* Files System: SD cards are typically formatted with the FAT16 filesystem, while SDHC cards are typically formatted as FAT32. (However, both types of cards can support other general-purpose filesystems, such as ext2 for example.)