What personal story about his own Guru did Sai Baba share with the determined old woman?

To soothe the old woman who was fasting for a mantra, Sai Baba shared a key story from his own past, which is recounted in Chapter 19. He explained that he also had a Guru, a great saint who was an ocean of mercy. Baba recounted how he had a strong desire to receive a secret 'ear-mantra' from his Guru and wore himself out in dedicated service to him. However, despite all his efforts and his vow to never leave his Guru's side, his Guru would not give him the mantra. By sharing this, Baba gave her the secret key of his Guruship, offering a deeper wisdom than the mantra she sought.