Duplicating a 10 GB file on a Mac can cost nothing, and that is correct
APFS clones share data between copies until one changes. What that means for backups, duplicates and the numbers your tools report.
Duplicate a 10 GB video in Finder and it completes instantly and the free space barely moves. Nothing is wrong: on APFS a copy within the same volume shares the original's data until one of them is modified.
What a clone is
The file system creates a second name pointing at the same blocks, and marks them copy on write. Both files are real and independent from your point of view. If you edit one, only the changed blocks are written, and only then does the space cost appear.
cp big.mov copy.mov # a clone, instant, near zero space
cp big.mov /Volumes/External/ # a real copy, across volumesThe distinction is the volume. Within one volume you get a clone; across volumes you get a genuine copy at full size, which is why the same action can take a second or ten minutes.
What this changes in practice
| Situation | What actually happens |
|---|---|
| Duplicating a large file | Near zero space until you edit one |
| Editing a clone | Space is used for the changed parts only |
| A duplicate finder reporting two 10 GB files | Deleting one may free almost nothing |
| Copying to an external drive | A full size copy, always |
| Time Machine local snapshots | The same mechanism, applied to the whole volume |
The duplicate finder trap
A tool that finds two identical files and offers to save you 10 GB is telling you about apparent size. If they are clones, deleting one frees almost nothing, which is a genuinely confusing outcome and not the tool lying. It is worth knowing before you subscribe to anything that promises space from duplicates, and it is one of the arguments in what duplicate finders can and cannot do.
How this relates to snapshots
A snapshot is the same idea at volume scale: a frozen reference to the blocks as they were. Deleted files stay on disk while a snapshot references them, which is exactly why deleting does not free space immediately.
Checking whether two files share storage
du -sh big.mov copy.mov # each reports the full size
du -sh --total big.mov copy.mov 2>/dev/null || du -ch big.mov copy.mov | tail -1Adding them with a total is the closest quick check: if the total is roughly one file's worth rather than two, they are sharing storage.
Common questions
Why does duplicating a file on a Mac use no space?
Because APFS clones it: the copy points at the same blocks as the original and only diverges when one is modified. Both files behave normally, and the space cost appears when you edit one.
Is a cloned file a real copy?
Yes for every practical purpose. It is an independent file that happens to share storage. What it is not is a backup, because both copies are on the same volume and share the same underlying blocks.
Why did deleting a duplicate not free space?
Because the two files were clones sharing the same data. Deleting one removes a name, not the contents, so the space is only released when the last reference to those blocks is gone.
Does copying to an external drive create a clone?
No. Clones only exist within a single volume. Any copy to another drive is a full size copy, which is why it takes time and space proportional to the file.