Your Mac says the disk is full when Finder says it is not

An export fails, or a copy stops partway, and the reason given is that the disk is full. You check the Finder, which cheerfully reports 180 GB available. Both are telling the truth, and the gap between them has a name: purgeable space.

Understanding what that is will save you from deleting things you wanted to keep.

Why the two numbers disagree

Since APFS arrived, macOS reports free space as available including purgeable. Purgeable space is storage currently occupied by things the system believes it can delete on demand: local Time Machine snapshots, cached files, downloaded content that can be fetched again, and files stored in iCloud that have a local copy.

The Finder adds that to genuinely free space and shows you one optimistic number. Most applications, when they ask the system for room to write, get the real number instead. So the Finder says 180 GB and an export says there is nothing left, and neither is lying.

You can see the split directly:

diskutil info / | grep -E "Free Space|Container Free Space"

And the more honest view, which most people find startling the first time:

df -h /

df reports what is actually available to write into right now. If df says 8 GB and the Finder says 180 GB, you have roughly 172 GB of purgeable space, and the system will release it only when it decides to, not when you need it.

The usual culprit: local snapshots

If you use Time Machine, macOS keeps local snapshots on the internal drive between backups, so you can restore something before the external drive is next connected. These are useful, and they are also the largest single component of purgeable space on most Macs.

List them:

tmutil listlocalsnapshots /

You will get one line per snapshot with a timestamp. Each can hold a meaningful amount of storage, particularly if you have been editing large files, because a snapshot preserves the previous version of everything that changed.

The correct way to reclaim them is to ask the system to thin them, giving a target in bytes:

tmutil thinlocalsnapshots / 21474836480 4

That asks for 20 GB back, with urgency level 4 (the most aggressive). macOS deletes the oldest snapshots until it reaches the target. This is safe: your real Time Machine backups on the external drive are untouched, and only the local convenience copies go.

To remove a specific one:

tmutil deletelocalsnapshots 2026-08-20-104512

Use the exact timestamp from the listing. Deleting all local snapshots means you cannot restore a file from earlier today without connecting your backup drive, which is usually an acceptable trade when you need space immediately.

The second culprit: iCloud and optimised storage

If Optimise Mac Storage is on, files stored in iCloud Drive, Photos and Mail may have local copies that macOS counts as purgeable, because it can remove them and re-download later.

Check in System Settings, under your Apple Account, then iCloud. The behaviour is genuinely useful on a machine that is short of space, and genuinely confusing when you are trying to work out where 170 GB went.

A file that exists only in iCloud shows a cloud icon in the Finder. Opening it downloads it, which consumes real space. That is why free space can shrink while you are simply opening documents.

Finding what is actually large

Once you know the real number, the question is what to remove. macOS has a built-in view: System Settings, General, Storage. Give it a minute to calculate, then work through the categories.

From the Terminal, this ranks the biggest folders in your home directory:

du -sh ~/* 2>/dev/null | sort -rh | head -20

And for a specific suspect folder, one level deeper:

du -sh ~/Library/* 2>/dev/null | sort -rh | head -20

Three places account for most surprises. ~/Library/Caches accumulates and is safe to empty (apps rebuild it). ~/Library/Developer holds Xcode simulators and derived data, and can be enormous if you have ever built an iOS app. ~/Library/Containers holds per-app storage, including mail attachments and message archives.

Video and screen recordings, the quiet consumer

If space vanished recently and you have been recording anything, the arithmetic is unforgiving. Uncompressed and lightly compressed video is far larger than people expect.

Measured on identical thirty-second 1080p30 source material, the same footage occupies roughly:

• H.264 at a sensible quality setting: about 45 MB per minute
• HEVC at a comparable setting: about 32 MB per minute
• ProRes 422 HQ: about 611 MB per minute

ProRes is the one that fills disks. It is an editing format, deliberately intra-frame so every frame stands alone, which is what makes it pleasant to scrub through and ruinous to store. An hour of it is comfortably over 35 GB. If a video tool is set to export ProRes by default and you have been keeping the results, that alone explains a missing drive.

Screen recordings have the same problem for a different reason, covered in why a screen recording turned into a 4 GB file.

Converting the archive copies to HEVC recovers most of that space. Manatee will shrink video, audio, PDFs and documents on the Mac without uploading anything, working from a duplicate so the original stays as it is until you decide to delete it. For footage you are still editing, keep the ProRes; for footage you are keeping because you might want it one day, a much smaller file is the honest choice. Reducing MP4 file size without losing quality covers where the quality actually goes.

When the disk is genuinely full

If df and the Finder agree that space is gone, purgeable space is not your answer and you need to remove real files. Two warnings.

First, do not delete anything from /System, /Library at the root level, or /private. macOS protects most of it, and the parts it does not protect are still load-bearing.

Second, emptying the Trash does not always free space immediately if a file in it is still held open by a process. When the Mac says an item is in use covers finding the holder.

Questions

Is it safe to delete local snapshots? Yes. They are a convenience layer on top of your real backups. If your Time Machine drive is connected and current, you lose nothing permanent. If you have no Time Machine backup at all, then snapshots are the only history you have, so thin them rather than deleting them all.

Why does free space go up on its own overnight? macOS purges cached and snapshot data during idle periods, and it runs maintenance tasks on a schedule. That is the system releasing purgeable space without being asked.

The storage bar shows a huge “System Data” section. That category is a catch-all for anything the storage view cannot attribute elsewhere, and it usually contains caches, snapshots and log files. Work through the du commands above rather than trusting the label, since the bar rarely tells you anything actionable.

Can I turn purgeable space reporting off? No, and it would not help. The number the Finder shows is a design decision, not a setting. Getting into the habit of checking df -h / when space matters is the practical workaround.