SPI Flash, File Update Problem

Hello,

I have a problem with RL-FlashFS for SPI Flash memory(AT45DB161). As you know, currently there is no 'update' option for NOR flash memory in the library. I can easly manage fopen, fclose,fdelete functions without problem. But when i try to update a file, i am using this;
- Read a file (fopen),
- Update some parameters in MCU-memory,
- Write them to the spi-flash back.

Sometimes, this sequence is not working perfectly. Especially, after 100bytes-file size, i faced many problems. 'fopen' function stores the stream in MCU-Memory perfectly and i can see the stream while debugging. But when i try to read (fread, fgetc or fgets), the file memory in MCU is gone.
If you help me, i will be appreciated... I added the code below;

U8 SDCardSetParam(char *fileName, char *param, char *value)
{
        FILE *fx;
        char name[100], fxBuf[1000];
        int i=0, index=0;
        PCHAR line[100];

        if(fileName == NULL)                            //No file name requesting...
                return 0;

        fx = fopen(fileName, "r");                    //Open the file for updating.
        if(fx == NULL)
                return 0;

        fread(fxBuf, sizeof(fxBuf), 500, fx); /*MCU memory is going bad here if the file is bigger than 100bytes. */
        fclose(fx);

        line[index++] = &fxBuf[0];
        while(fxBuf[i] != '\0')
        {
                if(fxBuf[i++] == '\n')
                        line[index++] = &fxBuf[i];
        }

        for(i=0; i<(index-1); i++)
        {
                if(sscanf(line[i], "%s\n", name) != EOF)
                        if(strcmp(name, param) == 0)    //Finding parameter line in the buffer.
                                break;
        }
        sprintf(line[i], "%s = %02X.%02X.%02X.%02X\n", param, *value, *(value+1), *(value+2), *(value+3));

        fx = fopen(fileName, "w");                    //Open the file for writing back.
        if(fx == NULL)
                return 0;

        fprintf(fx, fxBuf);
        fflush(fx);
        fclose(fx);

        return 1;
}