Look for a document on the Sandisk website called
"SDDK-05.R1.doc". It describes the SDDK for the SD driver
and filesystem used on the SD cards. It describes the functions
of the various libraries in the DDK, however does not give
any source code to those libraries. I priced the cost of the
full SDDK-05 early this year, and was quoted AU$10,000.

Some people in an Atmel contest run by Circuit Cellar
built a FAT16/32 filesystem driver for AVR.
http://www.circuitcellar.com/avr2004/d...nctive.htm

These files may be useful. The Sandisk files are hidden
on their server.

Sandisk SD card product manual - this describes the commands
to write and read data from the card.
http://www.sandisk.com/pdf/oem/P...rdv1.9.pdf

MiniSD card
http://www.sandisk.com/pdf/oem/P...SDv1.1.pdf

Older SD card
specshttp://www.sandisk.com/download/...rdv1.7.pdf

Application Note
Using SDCard and SDIO with the Intel® PXA250 MMC Controller http://www.intel.com/design/pc...33-001.htm

Toshiba SD card specification
http://i.cmpnet.com/chipcente...rod055.pdf

You can order the mmc card spec (version 3.31) for US$500 from www.mmca.org
Version 4 has been released.

SD cards are backward compatible with mmc cards.

These links may also be of some use.
http://instruct1.cit.cornell.edu/courses/e.../final.htm
http://homepage.ntlworld.com/seanellis...serial.htm
http://www.positron.org/projects/juicebox/
http://www.compsys1.com/workbench...oject.html