The continuing capacity ramping of Secure Digital cards will continue

basically uninterrupted through the next few years, as the flash card

vendors introduce yet another extension to the SD specification, called Secure Digital Extended Capacity. This week, Toshiba announced

the first card to launch on the new specification, a 64GB module

expected to ship next spring. Prices have not been announced, but the

new card capacities typically ship at prices slightly higher per unit

capacity than existing cards, so it's reasonable to speculate that a

cost of about $100-$150 is likely. Panasonic and PreTec had previously

SDXC announced cards with unknown launch dates.

The original SD spec was limited to 4GB card sizes (and 2GB and 4GB cards had compatibility problems with older readers), so in 2006 an extension was introduced, boosting maximum capacity to 32GB with a switch from FAT16 to the FAT32 file system and certain other minor tweaks. The SDHC spec could be sized up to 2TB without causing address size issues, but is limited by the spec to 32GB, a capacity first achieved last year.

Reading the new cards will require SDXC-compatible readers, which are likely to be available cheaply in USB interfaces and ship in lots of laptops at about the time cards appear. While this card boasts read speeds of 60MBps and write speeds of 35MBps, SDXC cards will eventually support read speeds as high as 300MB/s, we're told, which is within the bandwidth capability of USB 3.0. Capacities will range up to 2TB, so the spec may last up to ten years before another replacement is needed.�

A 2TB card would take about two hours to empty or fill at 300MBps, so it's possible that further enhancements in read speeds are due.

The new spec also brings a new file system to SD, exFAT, an extension of the FAT32 system with longer addresses, support for larger files and more granular logging, and other novel features. ExFAT, like FAT16 and FAT32, is owned by Microsoft and is more suited for memory cards than heavier-overhead file systems like HFS+ and NTFS. At present, exFAT is supported by Windows CE, Windows Vista, Windows 7. A patch is available to enable exFAT support on Windows XP, and an exFAT-supporting Linux kernel is under development. It's unknown when, if ever, OS X will support exFAT, but presumably it will happen eventually. If not, mac users would be unable to use SDXC cards.