SD cards can be used a couple of ways.
You can copy files to them.
But they also support DRM (digital rights management).
That allows a company like Garmin, to store
a GPS map file on the SD, and prevent
their customers from copying it. The Garmin
device, may have the key to open and access the
file, while regular file copies fail to work
properly.
And that is the function, most people
are interested in breaking.
"The "Secure Digital" component of the SD memory card
is a copyright protection scheme built into the chip.
Content Protection for Recordable Media (CPRM) is
intended to keep protected material from being
excessively copied."
http://en.wikipedia.org/wiki/Secure_Digital_card
"The digital rights management scheme embedded in the
SD cards is defined as the Content Protection for
Recordable Media (CPRM) by the 4C Entity and is
centered around use of the Cryptomeria cipher
(also known as C2). The specification is kept
secret and is accessible only to licensees."
http://en.wikipedia.org/wiki/Cryptomeria_cipher
HTH,
Paul