Secure Digital cards have been implemented in more than 8,000 devices, according to the SD Association. Innovations like SanDisk's Secure Digital High Capacity format and tiered transfer speeds drive the SD format forward, as can be appreciated when comparing standard SD cards with the company's progressive SDHC Ultra.
Note on SDHC Ultra
SDHC Ultra cards are variants of the SDHC format. SDHC Ultra cards differentiate themselves from standard SDHC card by meeting SD specification 3.0, which enhances their transfer speeds. The boost in data transfer rates makes them more reliable in transferring high-definition video, which could suffer dropped frames if a minimal transfer rate isn't substained.
Devices fitted for SanDisk's SDHC Ultra cards are backwards compatible with standard SD cards, though the inverse isn't true. Standard SD devices will only accept the original SD memory format. SDHC Ultra and standard SD cards share the same form factor, but they store data using different file systems.
Speed and Capacities
Standard SD cards come in capacities up to between 128MB and 2GB. SDHC cards are available ranging from 4GB to 32GB capacities. Standard SD cards have no defined specifications for their speed, while SDHC Ultra cards are capable of data transfer rates of 10MB/s on the low end at maximum rates of up to 30MB/s, according to SanDisk.
Standard SD devices are can only read the File Allocation Table 16 file systems used by standard SD cards and are incapable of reading the FAT32 file system used by SDHC and SDHC Ultra cards. The FAT32 standard awards files system more efficient use of space and allows SDHC cards to surpass the 2GB capacity limits of standard SD cards.