Secure Digital memory cards are a type of solid-state, non-volatile data storage media format designed for portable devices. SD cards are used by a wide variety of consumer electronics, including digital cameras, cell phones and PDA devices. They also come with both a write-protect option and the ability to enforce copy protection using Content Protection for Recordable Media, or CPRM, technology.
Standard Formats: SD
Memory cards made in the original Secure Digital format can be identified by the “SD” logo on the face of the card. Standard format SD cards were designed with a storage capacity limit of 2 gigabytes of data for all SD sub-formats. Standard SD cards use the FAT16 and FAT32 file system, which is the same file system that was used in Microsoft MS-DOS and Windows up to Windows XP. They can, however, be formatted to use other file systems. This format also includes the miniSD and microSD sub-types. The miniSD and microSD sub-types are used by more compact devices that desire to work with smaller card formats. Older devices that read SD cards are not necessarily compatible with later model formats such as Secure Digital High Capacity, or SDHC, and Secure Digital eXtended Capacity, or SDXC.
High-Capacity Formats: SDHC
All SDHC card formats have a maximum capacity of 32 GB of storage and use the FAT-32 file system. They are identified by the “SD” logo with the “HC” text inverted. Devices that use the SDHC card format are backward compatible with the original standard SD class of cards. Like the SD card, SDHC cards can be formatted to use other file system types. MiniSDHC and microSDHC cards are the same size as standard miniSD and microSD cards.
SDXC cards universally have a potential capacity of 2 terabytes of data storage. The full-sized SDXC card is the same size as the full-sized SDHC and SD card formats, and devices that use SDXC cards are backward-compatible with SD and SDHC cards of the same physical size or smaller sizes when used in adapters. SDXC cards use the Microsoft exFAT file system format, but can be formatted to use any other operating system that can handle 2 TB of storage.
Speed Class and Ratings
Another way to classify SD cards is by how fast they transfer data to and from consumer devices. According to SanDisk, speed classes for SD and SDHC cards are defined by a range of numbers from 2 to 10. Speed ratings are defined by data throughput: for instance, a speed class of 2 means a data transfer speed rating of 2 megbaytes per second, while a speed class of 10 means a speed rating of 10 MB per second. For SDXC cards there is the Ultra High Speed bus speed class, which Kingston Technology indicates has a current maximum speed rating of 104 MB per second at a speed class of 10.