rl-flashfs : fputs/fgets doesn't works

Hello,
I'm trying to setup the flash-fs library with SPI SD-card on STM32F100 device.
I've modified the SPI_STM32F100x.c driver to match with SPI2 device and needed I/O (CS, SD detect..)

The finit(), fformat() and fopen() functions are working. I'm able to format the card, create new files or open an existing file.

The problem comes when I try to write data into a file : nothing is written on the file. However, the fputs() function I'm using is returning 0 (OK).

I'm using RTX and the flash-fs library is used in a task :

/* Wait until the Card is ready*/
retv = finit ("M0:");
if(retv == 0)
{
        // Card initialized sucessfully

        // Open/create output file
        fout = fopen("M0:\\test.txt","w");
        if (fout != NULL)
        {
                // Write text in the file
                salut = fputs("fhghr", fout);

                // Close file
                fclose (fout);
        }
}
else if (retv == 1)
{
        // Card not initialized
}
else if(retv == 2)
{
        // Card is not formatted
        retv = fformat ("M0:Breitling /FAT32");
        if (retv == 0)
        {
                // Format OK
        }
        else
        {
                // Cannot format
        }

Can someone try to help me?

Thanks and bests regards

Michaël