The disk that won't empty: APFS snapshots and Time Machine

The scenario

It is a familiar panic. Your Mac warns that the disk is almost full. You hunt down an old folder of video exports, 40 GB of them, drag it to the Trash, and empty the Trash. You open the storage screen expecting relief, and the free-space figure has gone up by maybe two or three gigabytes. The other 37 are simply gone, not from the disk, but from the accounting.

This is one of the most common storage mysteries on a modern Mac, and almost every time the answer is the same: a snapshot. Understanding snapshots is the single most useful thing you can learn about how your disk reports space, because it explains not just this problem but half of what the storage screen shows you.

What a snapshot actually is

Your Mac's drive is formatted with APFS, Apple's file system. APFS is built around a principle called copy-on-write: when something changes, the file system does not overwrite the old data in place. It writes the new version to a fresh location and updates its records to point there. The old blocks are left alone until they are genuinely no longer needed.

A snapshot takes advantage of this. It is a read-only record of the file system at one instant. The record itself is tiny, just metadata. What makes a snapshot "cost" space is that it pins every block that existed at that moment. As long as the snapshot is alive, APFS is not allowed to reuse those blocks, even for files you have since deleted.

Mike Bombich, who makes Carbon Copy Cloner, lays out the arithmetic cleanly. Start with a 100 GB disk holding 80 GB. Take a snapshot. Delete 20 GB. The disk still reports 80 GB used: 60 GB of files you can see, plus 20 GB the snapshot is holding on your behalf. That 20 GB comes back only when the snapshot is removed.

Where the snapshots come from

You did not create any snapshots, so where did they come from? Time Machine. When you turn on Time Machine, macOS does not only back up to your external drive. It also takes local snapshots on your internal disk, so you can browse recent versions of files even when the backup drive is not connected.

These local snapshots are created on a schedule, and macOS manages them for you. In practice you can expect roughly one per hour of your startup volume, kept for about 24 hours and then removed automatically, plus a snapshot of the last successful Time Machine backup and one taken before any macOS update.

The important thing is that this is normal, designed behaviour. Local snapshots are meant to be invisible. They expand into free space when you have it and get cleared when you need it. The problem only becomes visible when you are already low on space and you expect a deletion to help immediately.

Why the free-space numbers disagree

Snapshots are the reason three different tools can show three different "free space" figures for the same disk at the same moment. This is not a bug; each tool is answering a slightly different question.

Howard Oakley, who writes the Eclectic Light Company blog and has documented APFS internals more carefully than almost anyone, has shown the Finder's estimate being wildly wrong, once claiming 227 GB available when the true figure was 144 GB, an error of 83 GB that exactly matched a bogus purgeable estimate. His standing advice is to ignore the Finder's free-space figure and trust Disk Utility or df. That is good advice, and it is why a serious storage tool should show you the honest number rather than the optimistic one.

How to see your snapshots

Open Terminal and run one command. It is read-only and completely safe:

# list local snapshots on the startup volume
$ tmutil listlocalsnapshots /

com.apple.TimeMachine.2026-09-03-081500.local
com.apple.TimeMachine.2026-09-03-091500.local
com.apple.TimeMachine.2026-09-03-101500.local

Each line is one snapshot, named with the date and time it was taken and ending in .local. If you see several, that is entirely normal. Their combined weight is whatever has changed and been deleted since the oldest of them was created.

How to reclaim the space, safely

There is a right tool for this, and it is not dragging anything to the Trash. The command tmutil thinlocalsnapshots asks macOS to purge snapshots until a target amount of space is reclaimed, and lets the system decide which ones to remove (oldest first).

# ask macOS to reclaim ~10 GB (10000000000 bytes), max urgency (4)
$ tmutil thinlocalsnapshots / 10000000000 4

# then confirm the space actually came back with the honest number
$ df -h /

The three arguments are the volume, the number of bytes to try to reclaim, and an urgency from 1 to 4, where 4 is the most aggressive. After it runs, check df -h rather than the Finder.

Is it safe to delete them?

Yes, with the right framing. A local snapshot is a convenience feature, not your backup. Removing one does not delete any of your current files, and it has no effect on the real Time Machine backups sitting on your external drive or network location. macOS itself deletes these snapshots automatically when space runs low; you are only doing early what the system would do under pressure.

What you should not do is go looking for snapshot files in system folders to delete by hand, or disable Time Machine entirely just to stop snapshots. Use tmutil, which understands what it is removing, or a tool that calls it correctly on your behalf.

A quick diagnosis checklist

Next time a deletion does not seem to free space, work through this in order:

1. Run df -h / and note the real free space, ignoring the Finder.
2. Run tmutil listlocalsnapshots /. If you see entries, snapshots are the likely cause.
3. Run tmutil thinlocalsnapshots / 10000000000 4 to reclaim roughly 10 GB, adjusting the number as needed.
4. Check df -h / again. If it has not moved, wait a few minutes or restart, then recheck.

That sequence resolves the overwhelming majority of "I deleted everything and the disk is still full" cases. The space was never really gone. It was promised to a snapshot, and now you know how to call it back.

Sources

• Mike Bombich, Carbon Copy Cloner documentation, Leveraging Snapshots on APFS Volumes — the 100 GB worked example.
• Howard Oakley, The Eclectic Light Company — APFS internals, purgeable space, and the inaccurate Finder free-space figures.
• Apple on-device manual pages: man tmutil for listlocalsnapshots and thinlocalsnapshots; man df.