Exercise 3: Why Only Two of Six Failures Are Actually About the Backup's Own Data — Possible Solution
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WHICH TWO ARE GENUINELY ABOUT THE BACKUP'S OWN DATA
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Per this chapter's warn-box, "only two ... a broken chain and a
genuinely corrupted file - are actually about the backup's own data."
A broken incremental chain means a specific piece of the backup data
itself is missing or damaged; a genuinely corrupted file means the
stored data is itself unreadable or invalid. Both describe something
actually wrong with the backed-up data.

WHY THE OTHER FOUR ARE ABOUT SOMETHING ELSE ENTIRELY
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Insufficient destination space is about the restore target's own
capacity, not the backup. A permission mismatch is about the
relationship between an account and the restored files' access
controls, not the files' content. Format/version incompatibility is
about whether the *current* restore tool understands an *older* (but
perfectly intact) format. A lost encryption key is about whether the
data can currently be decrypted, not whether the encrypted data itself
is sound. Network interruption during a transfer is about the
connection between two points, not about anything stored at either
end. In each case, the backed-up data itself could be entirely fine
while the failure still occurs.

WHY THIS DISTINCTION CHANGES HOW A TECHNICIAN SHOULD RESPOND
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If a restore failure is treated as proof the backup itself is bad, the
natural response is to abandon that backup and look for another one -
potentially discarding the only good copy over a problem that has
nothing to do with its actual integrity. Recognizing that four of the
six failure modes are environmental means the correct first response
to most restore failures is to investigate and fix the environment
(free up space, correct permissions, use a compatible tool version,
locate the key, retry the transfer) rather than concluding the backup
itself needs to be replaced or is unusable.

WHY THIS TIES BACK TO THE CHAPTER'S OWN WARN-BOX DIRECTLY
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This is precisely why the chapter instructs "check the environment
first; concluding 'the backup is unusable' too early can mean
discarding a perfectly good backup over a fixable local problem" - the
2-versus-4 split is the concrete evidence behind that instruction, not
just a general caution.

WHY THIS WORKS AS AN ANSWER
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It identifies the two genuinely data-related failures and explains why
each of the other four is about something separate from the backup's
own content, then explains the practical consequence: most restore
failures should prompt investigating the environment before concluding
the backup itself is at fault.