Summary
To back up photos from a Macbook Pro onto an external hard drive, make sure the external storage device is formatted for Mac, either APFS format or Mac OS Extended (Journaled) format.
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It is important to back up the library either by using Time Machine or manually copying the library to an external storage device.
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Additionally, any image files stored outside the Photo library should be backed up separately.
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Finally, a step-by-step guide on how to move an iPhoto Library to an external hard drive is provided.
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According to