Erwärmen said that Ferdinand rejected the knowledge, so I don't think it was a lack of mana that stopped the memory transfer partway through. And with how quickly and jumbled everything went through Rozemyne's mind, I find it hard to believe he would be able to know that the knowledge he looked for has already been transfered.

I think it's more likely that his curiosity just got the better of him and he barged into the place, without knowing what that would mean initially. And when he realized that he was well on his way to become the next Zent, he forecfully severed the connection, stopping the process cold. Remember that at the time Ferdinand was name-sworn to his father and had given the promise to always serve him and Ehrenfest first and foremost. If he had obtained the full Grutrissheit, that might well have been a breach of that promise and outright killed him on the spot. In light of that, I can easily see how even the mad scientist himself would block himself off from all the juicy knowledge.