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Secure Digital, or SD, is flash memory data storage device based on Toshiba's earlier Multi Media Cards (MMC). SD is slightly thicker, and includes features that allow the secure exchange of data, enabling usage restrictions to placate copyright holders. It had been said that Secure Digital cards would become ubiquitous, replacing diskettes, but since has failed to do so, largely because of the popularity of USB Keydrives. Its price is still prohibitively high, costing more and holding less data then its main competition, the larger Compact Flash. SD Memory Cards are usually faster than MMC cards and comparable to Compact Flash in terms of speed. The reason for that speedup is simple: MMC cards used 2 data lines wihle SD cards have 4.
PDA devices such as Pocket PCs and Palm-powered devices frequently feature SD slots. PalmOne, HP, Dell, Toshiba, and other PDA manufacturers have made SD a standard on all of their PDAs. Many of the Sharp Zaurus models also use the SD format. SD is fairly well supported in the digital camera industry as well, used in all Kodak cameras. Olympus also produced some cameras with SD capability, but abandoned it in favor of its own xD-Picture cards.
The SD standard is quite strict. It is not permitted to write open source drivers for SD card access. You can't write closed source drivers for yourself unless you become a SD Associacion Member to receive the specifications of SD cards ( 1500 $ per year).
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