Mac disk space: what "System Data" and purgeable space really hide

How to read General > Storage properly

Open Apple menu > System Settings > General > Storage. The colored bar splits used space into categories: Applications, Documents, Photos, macOS, System Data, and others depending on how you use your Mac (iCloud Drive, Mail, Messages, iOS Files).

Three things to know before drawing conclusions:

• The calculation takes time. When the pane first opens, a large share of space is often lumped into System Data because it hasn't been classified yet. Wait a minute or two and the categories sharpen.
• The macOS category is the system volume, sealed and read-only since Big Sur. Expect 12 to 20 GB depending on the version. You can't, and shouldn't, touch it.
• The recommendations below the bar (store in iCloud, optimize storage, empty Trash automatically, reduce clutter) are useful but generic. The info button next to each category shows its largest items.

"System Data": the great catch-all

System Data gathers everything macOS doesn't file elsewhere. It's normal for it to weigh 15 to 30 GB. Beyond 60 or 80 GB, there's usually a specific cause. The usual suspects:

• Time Machine local snapshots (see below), often culprit number one.
• Caches from apps and the system, in ~/Library/Caches and /Library/Caches.
• Logs and diagnostic reports, which can balloon if an app keeps crashing.
• Developer data: Xcode DerivedData and simulators, Docker images, npm, Homebrew or pip caches. On a developer's Mac this easily reaches 50 GB.
• Local AI models downloaded by tools such as Ollama or LM Studio, several GB each.
• Virtual memory (swap files) and the sleep image, managed by the system.

To see what's really hiding in there, you need a tool that shows the disk as it actually is, folder by folder.

APFS: container, volumes and clones

Since High Sierra, Macs have used APFS. Your disk is a container holding several volumes: the system volume (macOS), the data volume (your files and apps), and a few technical volumes (Preboot, Recovery, VM). These volumes share the same free space: there's no fixed partition. Disk Utility shows this when you choose View > Show All Devices.

APFS can also clone files. When you duplicate a file on the same volume, no data is copied: both files share the same blocks, and only later changes take up extra room. The practical consequence: Finder shows the full size of each copy, so the sum of file sizes can exceed the space actually used. Deleting a clone frees almost nothing as long as the original exists. That's also why a serious duplicate finder has to be careful with the numbers it reports.

Local snapshots and purgeable space

Local snapshots

When Time Machine is turned on, macOS creates a local snapshot of the data volume roughly every hour and keeps it for 24 hours, even when the backup drive isn't connected. macOS also creates one before every update. To see them, open Terminal and type:

tmutil listlocalsnapshots /

A snapshot copies nothing when it's created, but it holds on to the blocks of files you delete afterwards. That's why erasing 40 GB of videos doesn't free anything right away: the snapshot keeps them until it expires.

Purgeable space

Purgeable space is what macOS can reclaim on its own when room runs short: local snapshots, rebuildable caches, local copies of iCloud Drive files if Optimize Storage is on. It's in use today, but macOS frees it automatically on demand. In practice this works well for small needs, less well when an app suddenly needs 30 GB in one go (a macOS update, importing a big video project).

Why Finder and System Settings disagree

Finder counts purgeable space as available: it assumes macOS will free it when needed. System Settings often counts it as used (inside System Data). Hence a gap that can reach several tens of gigabytes. Add APFS clones, which Finder counts several times in folder sizes, and the delayed calculation in System Settings: both numbers are "true", they just don't measure the same thing.

The figure that matters for an update or a big file is real free space. When in doubt, Disk Utility shows the container's used and free space separately.

Freeing up space, in practice

In MacNeat, each module tackles part of the problem:

• Panorama displays the disk or a folder as proportional bubbles: one click to drill down, and the big consumers jump out, including inside System Data.
• Archipel finds large files, duplicates, similar images and forgotten downloads. See also our article on duplicate files.
• Clarté takes care of caches, logs, unneeded AI files and local Mail attachments.
• Nimbus spots iCloud Drive, Google Drive, OneDrive or Dropbox files stored locally and lets you evict them: they stay in the cloud but no longer take up room on your Mac.
• The Time Machine Snapshots tool in Atelier lists local snapshots that are reserving space.

Everything goes to the Trash, each operation is logged and can be undone, and scanning is free (deletions require a license). For a step-by-step method, follow our guide on freeing up space on a Mac.

What not to delete

• The contents of /System and system folders in general.
• The files in /private/var/vm (virtual memory).
• The inside of your Photos library or Music library: manage those from the apps themselves.
• The ~/Library/Mobile Documents folder: that's iCloud Drive, and deleting here deletes everywhere.