There are a number of reasons why I am not willing to agree with Schopenhauer. The fact that I am not willing to agree is, ipso fact, proof that I have free will!
Perhaps there are three types of reasons why I don't agree with Schopenhauer.
The first set of reasons has to do with the logic of determinism. The fact that we can predict an action, even with almost complete certainty, doesn't necessarily mean that the action is determined. I don't accept that, because at the point of action the subject could not have acted differently, determinism is true. If there is a formal possibility of the agent to act in a number of ways prior to the action, then there is the possibility of a non-deterministic explanation. We can envisage a sate of affairs where the agent acted differently.
The second set of reasons is based on what we mean by the term "free will". Schopenhauer says we can "act what we will" but we cannot "will what we will". This is just wrong in my view. My counter argument is effectively that we can "will what we will". We can will what we will because that is what we mean by the term "will". For you to prove that determinism is true you would need to prove to me that humans don't make choices - for me that is the essence of what I mean by determinism, and Schopenhauer has not proved that this is the case.
Of course you can redefine what I mean by free will and then say that reality doesn't accord with that definition, but all you have achieved is to play semantics with the definition.
The final set of reasons is to do with my conception of what it is to be an independent person or conscience. Schopenhauer has come up with one way of looking at human behaviour, but it doesn't cohere with my conception. Following an action back to its cause seems a very odd way of looking at human actions. When we talk about moral or normative human actions we use the concept of choice, we talk about blame and praise and human responsibility. I don't accept that all of these concepts are either meaningless or mistaken. I don't think the concept of causation (on its own) works very well in the realm of human actions. Human behaviour is not mechanistic, that is the essence of being human. What Schopenhauer is doing from my perspective is redefining what it is to be human.