What to do when a Time Machine copy to an external drive is enormously larger than expected
Time Machine uses a clever technique to make snapshots of your Mac backups. Instead of just creating copies of files that have changed since the previous backup—incremental archiving—Time Machine in macOS creates effectively a copy of each backed-up volume. The clever part is that it doesn’t duplicate any file that’s remained the same between snapshots; it just adds the newly updated files.
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