Comparisons & alternatives · Files, links and archive formats
Why doesn't duplicating a big file in Finder use more space, but copying it to another drive does?
Short answer: On APFS, a Finder Duplicate is a clone that shares the original's data, so it uses almost no extra space at first. Copying to another drive writes the full size, because nothing can be shared across drives.
On an APFS drive, Finder's Duplicate (and copying within the same volume) makes a clone: the copy shares the original's data until one of them is edited, so it uses almost no extra space at first. Copying to a different drive can't share anything, so the full size is written there.
Two consequences worth knowing:
- Deleting one of a pair of clones frees little or no space, because the other still holds the shared data.
- Edits to a clone gradually use real space, as the changed parts are stored separately.
This is also why moving a folder to your external drive only frees space on your Mac once the original is gone — the data has to physically live on the other drive. See why free space didn't go up after a move.