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First Aid from the top or bottom?
When running Disk Utility’s First Aid, which should I check first: the disk, or its volumes?
Conventional best practice argues that, before you verify or repair volumes, you need to know there’s nothing wrong with the disk’s partition table. Following that, you should run First Aid on the disk first, then on each container, and finally on each volume within the containers.
Maybe Apple File System (APFS) is different though: Apple has consistently recommended applying Disk Utility’s First Aid on volumes to start with, and once they’re repaired, checking containers, leaving the disk’s partitioning until last. For more info, this is described explicitly at www.apple.co/3kqloRT.
One important feature that Apple doesn’t mention, which may influence its recommendation, is that running First Aid on a volume also checks all of its snapshots. For volumes storing Time Machine backups, that’s time–consuming, and better done before checking that container. Problems are also most likely to occur in volumes, and less so in the disk’s top–level partitioning, so working from the bottom up does indeed make sense.
Disks still using the older HFS+ format can be checked and repaired either way, and don’t have containers in any case, as HFS+ volumes are partitions in their own right.
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