That particular warning, failed to copy external files, is telling you that in your topics you have links to topics that are outside your project. Often this is caused by linking to an output file or a topic in some other project.
Because users will not have access to anything on your C drive, the target is not included in the generation.
What you need to do is use Find and Replace and select Source Code, Then search on the file names reported. That should find those links so that you can correct them to become relative paths within the project.
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