Just one theoretical thinking. Let's assume we have two hard drives. A will be cloned to B. Let's furthermore assume that there's already data on B and it is intended to become overwritten during cloning. But on A there are some bad blocks which cannot be read. What will be on the clone (B) on those very sectors?

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Does your cloning software write to sectors on the target drive when the corresponding read has failed?

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You don't have to backup all of your files, just the ones you want to keep.

Yes, it depends what imaging hardware/software you are using. Always best practice to wipe destination drive prior to imaging.

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Data Recovery Ireland

Most cloning tools will fill unwriten sectors on destination with a specific pattern. But reading the cloning software manuals might help