Did Saint Galgano drive the sword into the rock?

No — and the difference is not a detail. The primary sources are the inquisitio of August 1185, that is the twenty depositions collected at Montesiepi four years after his death, and the earliest biographies: all of them say that Galgano planted the sword in terram, in the ground, in the manner of a cross. The rock appears later. It is a resemanticisation, and that stone is precisely the one element that would allow Galgano to be linked to the sword of King Arthur. Without it the link falls, and with it fall the two props that usually hold it up: the etymology Galgano → Galvano, which no philologist supports (Welsh Gwalchmei against Latin Galganus), and the identification of William of Malavalle with William X of Aquitaine, untenable because the latter died in 1137 at Compostela — a 17th–18th-century hagiographic invention already rejected by Baronius and the Bollandists. And there is an asymmetry that ought to be enough on its own: Arthur draws out, Galgano drives in. One receives a power, the other renounces it. The most recent monograph on the motif — Francesco Marzella, Excalibur, Salerno 2022, with a preface by Franco Cardini — locates the source of Robert de Boron’s Merlin elsewhere: in the crozier that Bishop Wulfstan of Worcester drives into the tombstone of Edward the Confessor, and that no one but him can pull out.