Ok, I went through my files and I found a shiur in which R’ Schacter said it. It’s entitled “Q and A – Tanach, Textual Differences in the Torah, Deciding Halacha, Choosing a Profession” and is available on yutorah.
If the moderators want to allow the link, it’s:
He discusses it at approximately 58:18.
Coincidentally, in the same shiur, at approximately 49:49, he says the story I quoted earlier demonstrating why you have to know all of Shas. It’s actually two separate stories which he says one after the other.