For purposes of this discussion, magic will be operating according to Discworld principles.
Magic is the principal force on the Discworld, and operates in a similar vein to real-world elemental forces such as gravity and electromagnetism. The Disc's "standing magical field" is essentially the local breakdown of reality which allows a flat planet on the back of a turtle to even exist. The force called "magic" is really just a function of the relative absence of reality in the local area, much in the same way that the absence of heat is described as "coldness." Magic warps reality in much the same way as the real universe's gravity warps its space-time. The act of performing magic is, essentially, telling the universe what you want it to be like, in terms it can't ignore.
The mana used to work magic operates on Star Wars principles which cribs from Eastern religions. It's an energy field produced by all living things and is part of a food chain for magical critters. So just like sunlight is captured by grass which is eaten by the deer which is eaten by man who is thus ultimately eating captured sunlight, the magic can be fed on as well. Some spirits can just suck it from the environment in passing like a filter feeder takes nutrients from passing water. This energy field just happens to be a byproduct of the life process. Animal eats food, burns oxygen and food, byproducts are CO2, heat, and mana.
So a wizard might directly draw upon local magic energy to make something happen or he could prevail upon some spirit entity to do the work for him. The less energy needlessly expended, magical and physical, the better off he'll be.
Presumably too much magic in one area could deplete the local "force" the same way oxygen can become depleted with too many people breathing in a confined space. This wouldn't make things die, it just means magic won't work until the the force/mana has balanced out again.
I would also say that a wizard channeling too much mana can "overheat" and require a cooldown. The magic power doesn't come from within him, he's a conduit.
One thought that springs to mind, if magic machines really are operating as paranormal Flintstones tech (see picture above) then this means that it's very possible for a machine to "give up the ghost" and break. A Maxwell's demon refrigerator might lose its demon and need to have another one coaxed inside. The owner will have to take good care of his equipment and keep the machine spirits happy or else it might not be agreeable. Nice divinator, good divinator. Please show me the weather forecast for tomorrow. That's a good divinator.
