SD cards have a maximum capacity of 4GB, while SDHC cards can go up to 32GB. Although this may seem like an extreme size, high-definition camcorders and digital photography enthusiasts have been pushing for a new format with higher maximum capacities and faster transfer speeds.
The industry answered earlier this year at the 2009 Consumer Electronics Show with the Secure Digital eXtended Capacity (SDXC) format, which has a maximum theoretical capacity of 2TB and could eventually reach speeds as high as 300 MB/s.
Although there were many promises made, there aren't really any SDXC cards that you can buy. A large part of the problem is due to the old chicken and egg paradigm; no one wants to produce a large, expensive, niche product if there are no SDXC card readers that can support the new format.
The SDXC format allows for backwards compatibility, meaning that readers should be able to use older SDHC, SD, and MMC cards. Development work has been slow, and the big push for products supporting SDXC won't occur until next year. The first wave of products will show up at this year's CES.
Many of those products will integrate SDXC card readers made by JMicron through a PCIe bus. JMicron's SDHC readers are commonly used in the industry and integrated into laptops, mobile phones, and digital cameras.
JMicron is also working on a standalone card reader that will be pluggable into a USB 3.0 slot. The company states that "PCIe or USB 3.0 interfaces are needed to unleash the full potential" of SDXC cards. Ultimately, read and write speeds will be determined by the flash memory card manufacturer, just as it is for SDHC cards.