There are two levels at least to the debates over the freedom of the will. The first has to do with the intersection of time and eternity, and is a problem that theists have to deal with. Arminians believe that God knows the future exhaustively, and Calvinists believe that He knows this future exhaustively because He foreordains all things. The question concerns how can a future event that will happen necessarily be freely chosen by its agents? That is one question. This is the macro-theology of the question. And Sam Harris is not talking about that.
But there is also a psychological aspect to it. Calvinists and Arminians differ on questions of time and eternity, sure enough, but they also differ when it comes to defining the will, and the freedom of it. Calvinists define free will as the uncontrained ability to choose in accordance with one's strongest desire. Arminians define free will as the unconstrained ability to choose contrary to one's strongest desire. This debate does not have to do with God's foreknowledge or foreordination (at least not directly), but rather with what free choice actually means on the invidual level.
This is necessary background to Sam Harris's second chapter, in which he (in his book on free will) shows himself to be blithely unware of the history of this debate. Jonathan Edwards laid this baby to rest in the 1700s, in his magisterial work The Freedom of the Will, and comes now Harris to use neureological experiments to show that the Arminians are wrong. But we already knew that.
The notion of free will that Harris is arguing against is the idea that at time T1 a conscious being can flip a switch in his consciousness in order to settle whether to go left or right. All the antecedent conditions of T1 do not settle the events of T2, but rather a raw choice made by the choosing agent.
But Harris points to certain neurological experiments that show that the brain has actually settled what is going to be done seconds before the actor is even aware of what he is going to do.
"These findings are difficult to reconcile with the sense that we are the conscious authors of our actions. One fact now seems indisputable. Some moments before you are aware of what you will do next -- a time at which you subjectively appear to have complete freedom to behave however you please -- your brain has already determined what you will do. You then become conscious of this 'decision' and believe that you are in the process of making it" (p. 9).
But talk about yesterday's newspaper. Edwards demonstrated that our conscious choices proceed from our heart, our nature, and he did this centuries ago. A free man, according to Calvinists, is a man who is not externally constrained, and has the ability to do what he wants. And what he wants is determined by far more than what can be settled by flipping coins in the control room. This is why Calvinsts prefer to talk about free agents, or free men. They do not like to talk about an autonomous free "will," floating around out there, detached from the heart. When you talk that way too much, someone like Harris might make an elementary mistake.
We make our choices out of the reservoir of our nature. Jesus taught this (Matt. 12:35). The mechanism of the will does not have the ability to alter the contents of our hearts. The nature of the tree determines the nature of the fruit produced.
If we could choose contrary to our strongest desire, we would not call that freedom, but rather insanity. "Why did you throw the vase against the wall?" "Because what I really wanted to do was to go for a walk."
The act of willing is a gift God gave us for identifying what we want to do. It was never intended to create what we want to do.