Re: The maths don't add up...
"More importantly, if a brief blip COULD have cost you 10% of the month's supply, why do you not have generators to stop that happening?"
The short answer - this isn't a datacenter, or even a car factory.
A chip fabrication line (fab) is a huge power user - typically 50-75MW. If this plant is big (as it appears to be) it probably has multiple active fabs - so any power source will not be that little diesel generator you have out back, it will need to be big enough to power a small city. It means spending probably $US40M+ per fab on those backup generators (back-of-envelope calculation - 60MW x ~US$700 per KW for big NG generators), which makes it a bit less of an obviously good idea. I'd have to see the actual cost of the shutdown to have an informed opinion.
If this is Samsung's only big NAND manufacturing facility, I'd say (as suggested in a previous comment) the bigger fault is having an "all eggs in one basket" supply chain.