There is a big difference, when you copy - you do not look at what you copy, you do not read what you copy, of course you do not understand what you copy. While, when you rewrite, you have to look, read, understand, learn. Sometimes you have to read it a few times before rewriting. Thus, you become smarter when you read more, write more, and not just copy and think. That is, do not strain your brains at all!
Maybe there is, I just did not know about it. Or maybe we are talking about different things. Can you please offer me materials that I could study in order to make my nocturne or other musical reproduction?