Back to Hume's point: the fact that 99.99% of people choose not to excersise it does not mean that free will does not exist. Neither does the fact that things happen which are beyond one's control.
The existentialist position is summed up as "man's existence preceeds his esscence." My cat here is a predetermined being. He is a hunter and a carnivore. He can not choose otherwise. In fact, it would be harmful to him if he could. Despite the fact that he has a ready source of food available for the taking that he need do no work for, he still exhibits predatory behavior and will still hunt down and kill things.
I, on the other hand, can be a hunter or a vegan. I can even be one thing one day and another the next. If a debilitating health condition forces me to abandon a long held career, I can allow it to devastate me emotionally and psychologically or I can view it as an opportunity to grow.
If the cat (assuming his natural state without the human intervening to do the deed for him) can no longer hunt, he dies. That simple.
Back to Neal Peart's point: even if you do not choose to exercise your free will, that in and of itself is a choice. We really have no choice but to choose. One way or another.
As Sarte put it: "man is condemned to be free."