Secure Digital (SD) is a flash (non-volatile) memory card format
developed by Matsushita, SanDisk and Toshiba for use in portable
devices, including digital cameras, handheld computers, PDAs and
GPS units. As of 2007, SD card capacities range from 8 MB to 16
GB. Several companies have announced SD cards with 32 GB. Cards
with 4-32 GB are considered high-capacity. The format has proven
to be very popular. However, compatibility issues between older
devices and the newer 4 GB and larger cards and the SDHC format
have caused considerable confusion for consumers. SD cards are
based on the older MultiMediaCard (MMC) format, but have a
number of differences: ?The SD card is asymmetrically shaped in
order not to be inserted upside down, while an MMC would go in
most of the way but not make contact if inverted. ?Most SD cards
are physically thicker than MMCs. SD cards generally measure 32
mm ?24 mm ?2.1 mm, but can be as thin as 1.4 mm, just like MMCs
(see below). ?The contacts are recessed beneath the surface of
the