- It defined electrically identical cards in smaller sizes: miniSD and microSD (originally named TransFlash or TF). Smaller cards are usable in larger slots through use of a passive adapter. By comparison, Reduced Size MultiMediaCards (RS-MMCs) are simply shorter MMCs and can be used in MMC slots by use of a physical extender.
- It defined higher-capacity cards, some with faster speeds and added capabilities: SDHC (Secure Digital High Capacity) and SDXC (Secure Digital eXtended Capacity). These cards redefine the interface so that they cannot be used in older host devices.
- It defined an SDIO card family that provides input-output functions and may also provide memory functions. These cards are only fully functional in host devices designed to support their input-output functions.
 |
| Size comparison of families: SD, miniSD, microSD |
Physical size
- Standard size
- SD (SDSC), SDHC, SDXC, SDIO
- 32 mm × 24 mm × 2.1 mm
- 32 mm × 24 mm × 1.4 mm (as thin as MMC) for Thin SD (rare)
- Mini size
- miniSD, miniSDHC, miniSDIO
- 21.5 mm × 20 mm × 1.4 mm
- Micro size
- microSD, microSDHC, microSDXC
- 15 mm × 11 mm × 1.0 mm
SDHC
 |
| 16GB SDHC Class 6 |
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 , 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.[1] 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 SP1 also requires a later service pack.
SDXC
The Secure Digital Extended Capacity (SDXC) format supports cards up to 2 TB (2048 GB), compared to a limit of 32 GB for SDHC cards in the SD 2.0 specification.- Compatibility with SDHC
- SDXC cards are pre-formatted with Microsoft's proprietary and patented exFAT file system, which the host device might not support. Since Microsoft does not publish the specifications of exFAT and its use requires a non-free license, many alternative or older operating systems do not support exFAT for technical or legal reasons. The use of exFAT on some SDXC cards may render SDXC unsuitable as a universal exchange medium, as an SDXC card that uses exFAT would not be usable in all host devices. Since the FAT32 file system supports volumes up to the SDXC's maximum theoretical capacity of 2 TB as well, a user could reformat an SDXC card to use FAT32 for greater portability between computers . FAT32-formatted SDXC cards can be used in a host device built for SDHC if the host device can handle 64GB and larger volumes.
- SDHC host devices will not test the new capability bits defined for SDXC 4.0 cards. It will therefore not be able to use the new features of SDXC, such as transfer speeds above UHS104 (104MB/s).
- Host Operating System support
Apple Mac OS X versions that support SDXC cards and exFAT are Mac OS X Snow Leopard 10.6.5 or later including OS X Lion 10.7.
BSD and Linux systems that support SDHC cards also support SDXC cards that contain a compatible file system, but they normally do not support the proprietary exFAT file system that is installed on SDXC cards, because of patent issues. The user may reformat the card to contain a different file system .
SDIO
SDIO
A SDIO (Secure Digital Input Output) card is an extension of the SD specification to cover I/O functions. Host devices that support SDIO (typically PDAs like the Palm Treo, but occasionally laptops or mobile phones) can use the SD slot to support GPS receivers, modems, barcode readers, FM radio tuners, TV tuners, RFID readers, digital cameras, and interfaces to Wi-Fi, Bluetooth, Ethernet, and IrDA. Many other SDIO devices have been proposed, but it is now more common for I/O devices to connect using the USB interface.SDIO cards support most of the memory commands of SD cards. SDIO cards can be structured as 8 logical cards, although currently, the typical way that an SDIO card uses this capability is to structure itself as one I/O card and one memory card.
- Host support for SDIO
