Having become lieutenant colonel from 16 November 1927, he was military attaché at the Italian embassy in the Weimar Republic and, in 1934, he became a teacher of German at the Royal Academy of Infantry and Cavalry in Modena, where he remained until 31 December 1935, when he was discharged from active service.

He was then imprisoned in Oflag 64/Z in Schokken, Poland, where he fell seriously ill with tuberculosis; in spite of his condition, he refused to an offer to be repatriated and cured in exchange for swearing allegiance to the Italian Social Republic.

In late January 1945, with the Red Army approaching, the Germans decided to evacuate the prisoners westward with a forced march on the snow; during the march, on 28 January, Ferrero was murdered by the SS for being unable to keep pace with the group, along with generals Carlo Spatocco, Alberto Trionfi, Emanuele Balbo Bertone, Giuseppe Andreoli and Alessandro Vaccaneo.

Ferrero was the last to be killed; General Ettore De Blasio later recounted his last words before collapsing and being shot had been "I can no longer walk, my foot is swollen, my legs cannot hold me.

Unlike the other generals, his body was never recovered; in May 1945 his family was wrongly informed by the Italian Embassy in Moscow that he was alive and well, but one month later they were notified of his death.