Something

$ grep -r cardWriteCommand /home/rfbaum2/source/devkitpro/libnds
Binary file /home/rfbaum2/source/devkitpro/libnds/lib/libnds7.a matches
Binary file /home/rfbaum2/source/devkitpro/libnds/lib/libnds9.a matches
/home/rfbaum2/source/devkitpro/libnds/include/nds/card.h:87:void cardWriteCommand(uint8 * command);

$ grep -r card.h fat
fat/io_nmmc.c:30:#include <nds/card.h>

$ make
arm-eabi-g++ -Wall -O2 -mcpu=arm9tdmi -mtune=arm9tdmi -fomit-frame-pointer -ffast-math -mthumb -mthumb-interwork -I/home/rfbaum2/source/devkitpro/libnds/include -DVERSION_NO=\"0.1.4b\" -DARM9 -c arm9_main.cpp -oarm9_main.o
arm9_main.cpp:248: warning: 'int EmulateScanBG3(unsigned int, short unsigned int*)' defined but not used
arm9_main.cpp:277: warning: 'int EmulateScan(unsigned int, short unsigned int*)' defined but not used
arm-eabi-g++ -mthumb-interwork -mno-fpu -specs=ds_arm9.specs arm9_main.o arm9_sound.o arm9_file.o -L/home/rfbaum2/source/devkitpro/libnds/lib -L../Pico/ -L./fat/ -lnds9 -lPico -lndsfat -o arm9.eabi
./fat//libndsfat.a(io_nmmc.o): In function `Neo_SelectMMC':
io_nmmc.c:(.text+0x29a): undefined reference to `cardWriteCommand'
collect2: ld returned 1 exit status
make: *** [arm9.eabi] Error 1