The files would become corrupt because the image would try to expand larger than the total file size possible on the snap server. That would then cause the data in the image to become damaged. I warn about it strongly in the User's Guide because the OS doesn't always notify about this situation, and can both silently corrupt files or even crash your Mac.
Depending on what's gone on here, if the files are truly corrupt they are not likely recoverable.
Is the size of the image file 4GB?
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--Dave Nanian
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