I have a flash drive, which is ten-percent full, but has been used extensively. (i.e. a large minority of blocks will contain random noise.)
In order to make a small, compressed image of this drive (if it were an HDD) I would dd if=/dev/zero of=/tmp/zeros.tmp bs=16M; rm /tmp/zeros.tmp. This fills all unused blocks with zeros, so that an image of the drive compresses well.
With a flash drive, would there be any advantage in writing 0xFFs to unused blocks, instead of zeros? i.e. is the flash drive clever enough to not rewrite a page that already contains 0xFFs?
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