A SD card will normally work in a SDHC device (possibly with lower performance). SD cards are in the following capacities 4,8,16,32,64,128,256 512MB and 1GB, 2GB only. Currently mainly 2GB SD cards are being sold.
A SDHC card will not work in a SD device such as a camera or reader. SDHC cards are in the following capacities 4GB, 8GB, 16GB and 32GB
SDHC (Secure Digital High Capacity) was established to meet the growing demand for HD (High Definition) video and high resolution image recording now used in many SD-enabled devices.
SDHC is the same physical size and shape as standard SD but meets the new SD specification of version 2.0. If the SD card is 4GB or above, it is classed as a SDHC card. Currently, SDHC specifications allow for memory cards of a capacity between 4GB and 32GB.
SDA (SD Card Association) has also established a Speed Class rating (Class 2, 4, 6 and 10) for the standardisation of data transfer speeds (compulsory for SDHC cards)
CLASS 2 – a guaranteed minimum write speed of 2MB/s or higher*
CLASS 4 – a guaranteed minimum write speed of 4MB/s or higher*
CLASS 6 – a guaranteed minimum write speed of 6MB/s or higher*
CLASS 10 – a guaranteed minimum write speed of 10MB/s or higher*
*The minimum speed capability depends on the manufacturer of the host device and model.
All Class 2, Class 4, Class 6 and Class 10 SDHC cards are compatible with SDHC devices. Please note that if the device manual suggests a SDHC card of speed Class 4 as a minimum, the user should install a Class 4, Class 6 or Class 10 (faster) SDHC, NOT a Class 2 card as this may reduce performance of the device.