#define DRV_SD0 0
#define DRV_SD1 1
#define DRV_SDRAM 2

...

DRESULT disk_read (
BYTE drv, /* Physical drive number (0..) */
BYTE *buff, /* Data buffer to store read data */
DWORD sector, /* Sector number (LBA) */
BYTE count /* Sector count (1..255) */
)
{
unsigned char result;
DRESULT res = RES_ERROR;
unsigned sectorSize = 0;

if (drv == DRV_SDRAM) {
sectorSize = SECTOR_SIZE_SDRAM;
}
else if(drv == DRV_SD0) { // вот эти вещи добавлены
sectorSize = SECTOR_SIZE_SDCARD;
}
else if(drv == DRV_SD1) { // вот эти вещи добавлены
sectorSize = SECTOR_SIZE_SDCARD;
}

result = MED_Read(&medias[drv],
medias[drv].baseAddress + (sector * sectorSize), // address
(void*)buff, // data
count * sectorSize, // data size
NULL,
NULL);

if( result == MED_STATUS_SUCCESS ) {
res = RES_OK;
}
else {
TRACE_ERROR("MED_Read pb: 0x%X\n\r", result);
res = RES_ERROR;
}

return res;
}

в medias[] лежат указатели соответствующие