How to back up an external hard drive to another

If all you want is a straight backup of one external drive to another, skip Time Machine, it only backs up volumes that are part of your Mac, not external-to-external copies.

The simplest way is to connect both drives, open Finder, and drag the entire top-level folder structure from Drive A to Drive B. If you want something safer and faster for large drives, use Carbon Copy Cloner or SuperDuper, they can clone an external drive exactly, handle errors properly, and let you update the backup later without recopying everything.

In short, Time Machine won’t help here, but Finder copy or CCC will get the job done cleanly.

Do you know about disk cloning or image creation? These are some other methods which might be an option for you.

Set up Time Machine, or use a product such as Carbon Copy Cloner, or manually drag files to it.

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Apple's very good backup system, Time Machine, is tightly integrated into macOS.

Back up your Mac with Time Machine - Apple Support

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