Did Sai Baba ever receive a secret mantra from his Guru? What did he teach about it?

No, Sai Baba did not receive a secret mantra, despite his strong desire for one. In Chapter 19, he recounts how he served his Guru tirelessly, hoping to receive an 'ear-mantra,' but his Guru never gave him one. Instead, Baba received something he considered more profound: immense love, constant protection, and the blissful experience of simply gazing upon his Guru. He used the analogy of a mother tortoise who nurtures her young from across a river with only her loving glance, making a mantra unnecessary. He explains, "my Guru did not blow any mantra into my ears, so how can I blow one into yours?" This illustrates that the Guru's grace and loving presence are more potent than any specific instruction.