SDHC cards are physically and electrically identical to standard-capacity SD cards (SDSC). The major compatibility issues between SDHC and SDSC cards are the redefinition of the Card-Specific Data (CSD) register in Version 2.0 (see below), and the fact that SDHC cards are shipped preformatted with the FAT32 file system.
Host devices that accept SDHC cards are required to accept SDSC cards. However, host devices designed for SDSC do not recognize SDHC or SDXC memory cards, although some devices can do so through a firmware upgrade. Older operating systems require patches to support SDHC. For instance, Microsoft Windows XP before SP3 requires a patch to support access to SDHC cards. Windows Vista also requires service pack 2.
For more information please click here