SD.exists(filename) works faulty!
If the SD has been started correctly by SD.begin(SD_CS) and the SD should be checked if intermediately mistakenly removed, it always shows "OK" and never shows an error:

 if( SD.exists("/") ) {
 Serial.println(" SD path '/' OK! "); // either if SD inserted or removed !!!
 // debug
 delay(2000);
 }
 else {
 Serial.println( "error! no SD file path found! "); 
 // debug
 delay(2000);
 }

Arduino IDE 1.8.9
ESP32 board 1.0.4