SD cards are based on the older Multi Media Card (MMC) format, but most (not all) are slightly thicker than MMC cards. These flash memory cards were originally designed to hold copyrighted musical content and prevent its unauthorized duplication. Although this use was not widely adopted, SD cards have become the flash memory of choice for digital cameras because of their small size and low power consumption. Small cameras need small memory cards and the larger CF cards were no longer viable due to their physical size. SD memory cards are physically smaller than SmartMedia cards but thicker. Most (not all) offer an optional lock switch on the side to prevent accidental overwriting, and (little-used) DRM security features. There are two main types available, ones that run at regular speeds, and high-speed cards that have higher data transfer rates. These cards range in size from 4MB up to 2GB. SD cards generally measure 32 mm × 24 mm × 2.1 mm, but can be as thin as 1.4 mm, just like MMC cards.
Devices with SD slots can use the thinner MMC cards, but the standard SD cards will not fit into the thinner MMC slots. SD cards can be used directly in CompactFlash or PC Card slots with an adapter. miniSD and microSD cards can be used directly in SD slots with an adapter. There are some SD cards that have a USB connector built in for dual-purpose use, and there are readers which allow SD cards to be accessed via many connectivity ports such as USB, FireWire, and the common parallel port.OpticsPlanet, Inc offers a full line of original Secure Digital Memory Cards with full manufacturer's warranty and Free Shipping on orders over $29.95!

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Lexar Secure Digital Memory Cards Magellan Secure Digital Memory Cards

SanDisk Secure Digital Memory Cards Silicon Power Secure Digital Memory Cards