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To avoid corrupted downloads I always use the torrents, because this download method verifys itself. The Knoppix torrents also contains checksums for checking.

I cannot help you with your other...

When booting Knoppix from the ISO file, the bootng process
1) searches for the ISO file (knoppix810.iso) in the root of the detected file systems
2) if the ISO file is found, it mounts the ISO file...