SD Memory Cards or also known as Secure Digital (SD) is a non-volatile memory card format developed for use in portable devices.
SD Memory cards is a popular flash memory card developed by Matsushita, SanDisk, and Toshiba for the use in portable devices. It is widely used in digital cameras, PDAs, mobile phones, GPS devices, and video game consoles. The standard size SD card comes in capacities ranging from 8 MB to 4 GB, while the SDHC (Secure Digital High Capacity) comes in capacities ranging from 4 GB to 32 GB. A full sized SD memory card measures 24mm x 32mm x 2.1mm, they are based on the older MMC (MultiMediaCard) format. Devices with SD Memory card slots can also be used with MMC cards, however since the SD memory card is thicker it cannot be inserted in to an MMC slot.
The secure digital format was first developed to compete with Sony's Memory Stick format. Panasonic, SanDisk, and Toshiba began development in 1999. The SD memory card first became available in the beginning of 2000. SD memory card have a number of main differences from the MMC cards which they were based on. SD memory cards are shaped asymmetrically to insure they are not inserted upside down, while an MMC card could be inserted but not make contact. SD memory cards are physically thicker, SD memory cards typically have transfer rates in the range of 10-20 Mbps, however this is constantly improving with todays changing standards.
Some SD memory cards include a USB device for compatibility with laptop and desktop computers. It allows the Secure Digital memory card to be connected to any computer with a USB drive. SD memory cards offer an optional write-protect tab. It is a simple slidable tab or switch located on the left side of the card. When the write-protect tab is in the down position (Away from the end that is inserted, the card is write protected meaning it is read only. The write tab feature is considered optional by Secure Digital Association Guidelines. Some manufacturers believe that the write-protect switch is easily broken. If the sensor of the device is not functioning properly and unable to detect the notch, all SD memory cards will seem to be either write-protected or write-enabled.
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History of the SD Memory Card
In August 1999, SanDisk, Matsushita, and Toshiba first agreed to develop and market the SD (Secure Digital)
Memory Card, which was a development of the MMC. With a physical profile of 24 mm × 32 mm × 2.1 mm, the new card
provided both DRM up to the SDMI standard, and a high memory density for the time.
The new format was designed to compete with Sony's Memory Stick format, which was released the previous year,
featured MagicGate DRM, and was physically larger. It was mistakenly predicted that DRM features would be widely
used due to pressure from music and other media suppliers to prevent piracy.
SD Memory Card File system
Like other flash card technologies, most SD Memory cards ship preformatted with the FAT or FAT 32 file system
on top of an MBR partition scheme. The ubiquity of this file system allows the card to be accessed on virtually
any host device with an SD Memory card reader. Also, standard FAT maintenance utilities (e.g. ScanDisk) can be used to repair
or retrieve corrupted data. However, because the memory card appears as a removable hard drive to the host system, the
card can be reformatted to any file system supported by the operating system.
SD Memory cards with 4 GB and smaller capacities can be formatted to either FAT16 (4 GB card can be formatted to FAT16 only
with 64k clusters) or FAT32 file systems. SD Memory Cards 8 GB and larger can only be formatted with a file system that can handle
these larger storage sizes, such as FAT32.
SD Memory cards are not limited to using only MBR partitioning and the FAT file systems. Under Windows, SD Memory cards can be formatted
using the NTFS and exFAT file systems; under Unix-like operating systems such as Linux or FreeBSD, SD Memory cards can be formatted using
f.e. the EXT3 or the ReiserFS file systems; under Mac OS X, SD cards can be partitioned as GUID devices and formatted with the HFS+ file system.