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SD cards are not delivered empty - they have a FAT filesystem by default (part 2 of the spec, not public). I could wipe the card, but it's really the same operation as overwriting it. Since I was overwriting the entire card sequentially the prior contents should not affect the test. I was not overwriting it with files but an image of the entire card contents. It was not mounted so no filesystem effects were involved. Basically like using Ghost or similar disk imaging tools in exact copy mode.
In short, yes, I overwrote it with the same contents. The results weren't the same, however, though I haven't analyzed precisely what it did; my guess is dropped data such that what remained got shifted to lower addresses.
Note that it appears a partitioned card is not exported as the whole card by the PocketBook, but as the partition itself. I avoided this difference simply by having the card unpartitioned.
Come to think on it, actually wiping by SD erase commands might be a good idea. I don't know if I have any device that can do that.
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