Winston -- v2.0 allows sparse images to be created, but it's not released yet. v1.5.5 only allows DMGs.
The whole thing is that a sparse image is just that: sparse. We can guess high when we create it, because only takes as much space as the data that's eventually copied to it. Guessing high when actually "testing" for size would mean we'd reject legitimate creation.
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--Dave Nanian
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