Thnx Nick Gammon
I recommend reworking your code to manage without String. Use C-style strings instead (strcpy, strcat, strcmp, etc.).
Ok. One more question
Is this is a way about which you are suggesting?
myFile = SD.open("config2.txt", FILE_READ);
Serial.println(myFile.size());
char* buf = (char*) malloc(myFile.size());
readed = myFile.read(buf,myFile.size());
Serial.println(readed);
myFile.close();