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From a post by CosmicPeter on Tom’s Hardware:
What's happening physically, is each time a block is written, there is a residual voltage buildup. Eventually, this voltage buildup gets so large, the applied voltage to re-write to the block is of comparable size. Thus the block can no longer be changed. So there is no definitive number of write times, nor is it something the manufacturer wants. There is currently a fair amount of research in how to reset or prevent this voltage buildup (which eventually causes the drives to fail, by becoming read only. This is one big advantage over a HDD).