auto reset during the loop()

Take a closer look at this section of your code:

    char numString[TAILLE_NUM_UTI];

    //place le pointeur au debut du fichier
    myFile.seek(0);
    //retrieve the ID number of the line
    for(i=0; i<TAILLE_NUM_UTI-1; i++) numString=myFile.read(); 
    numString='\0';

Specifically:

   char numString[TAILLE_NUM_UTI];
   for(i=0; i<TAILLE_NUM_UTI-1; i++) numString=myFile.read(); 
  numString='\0';

Last I checked, read() doesn't return a pointer, it returns a single byte. That shouldn't even compile...