How can I make raisins really hard & chewy?

I think you just need to dehydrate them some more to get that effect. I assume granola and muesli tend to use drier raisins because they will keep longer, and not affect the rest of the cereal with extra moisture. The amount of moisture left in raisins can be variable, it depends on how they're made and for what purpose - the moister ones are more accessible, the drier ones keep longer - that would be why some are softer than others.

To dry out your raisins, you might sun-dry them, or use a dehydrator, or even lay them out in a warm oven to chase of the extra moisture. You would have to keep checking to figure out when they reach the consistency you want. You might even try toasting them, for a higher temperature and a quicker time - though I'm not sure if the higher heat will change the flavors, or if the changes would be any good... I have not heard of toasted raisins as an ingredient.

I think cooking the raisins is the trick. Try sautéing them the raisins in oil over medium heat, stirring frequently until they are puffy and round. Let them cool, and you will be left with extra chewy raisins, similar to the texture of ones in a granola bar.