They’re some of the commonest actions in the Finder: copying and moving files around within and between volumes. This primer explains how they work on APFS volumes in Catalina, where they aren’t as simple as they used to be in HFS+.
Key concepts
In APFS, volumes behave differently from the way that they do in HFS+. Although some compare them with traditional folders, that’s misleading and will only confuse. Each volume has its own file system, complete with directories and other metadata, including extended attributes. Volumes within the same container share the same space on disk, so they can flexibily grow and shrink sharing the same free space. But they don’t share file systems: drag a file from one volume to another and a new copy of that file is made on the second volume. You can’t share data or inodes between APFS volumes, although inodes are managed differently across Catalina’s new Volume Groups.
iCloud storage is mapped to files on your startup (Data) volume, and effectively forms part of that volume even when all those files have been evicted to remote storage. When a file is evicted to iCloud, it’s still represented by a local stub file. The important consequence of this is that iCloud Drive behaves as if it’s an extension of your startup volume, not a different volume altogether.
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