What is System Data on Mac, and how to shrink it safely

What System Data actually is

Apple describes System Data (called Other on older macOS) as a general category for files that don’t belong to any more specific one, like Applications, Documents or Photos. It’s worked out as a remainder: macOS adds up the named categories and whatever is left over becomes System Data. That’s why the number is rough, why it jumps around, and why there’s no Delete button for it.

What’s usually inside it

• Caches. Browsers, Xcode, Homebrew, package managers and most apps keep caches in ~/Library/Caches. Many grow for years and are rarely cleaned up.
• Time Machine local snapshots. If Time Machine is on, macOS keeps snapshots on the internal disk too. macOS removes them by itself when space runs low, but until then they can hold a lot.
• Developer data. Xcode’s DerivedData, simulators and device support files, Docker Desktop’s disk image, Android emulators and SDK images, and npm, pnpm, Gradle or pip caches. On a developer’s Mac this is often the biggest part.
• iPhone and iPad backups made through Finder, stored in ~/Library/Application Support/MobileSync/Backup.
• Leftovers of deleted apps. Dragging an app to the Trash leaves its data behind in ~/Library (Application Support, Containers, Caches, Preferences).
• Logs, virtual memory and purgeable space that macOS manages itself.

What not to do

Don’t delete folders under /System, /private/var or /Library because a forum post said they’re big, and don’t run rm -rf on things you haven’t checked. Some of what counts as System Data is exactly what keeps macOS and your apps working. Clear specific, known things, and prefer moving them to the Trash so you can put them back.

Safe ways to shrink it by hand

• Empty the Trash. Space from trashed files only comes back once the Trash is emptied.
• Check local snapshots. Run tmutil listlocalsnapshots / in Terminal to see them. macOS thins them automatically, so usually you can just wait. Delete one with sudo tmutil deletelocalsnapshots and its date only if your real backups are fine.
• Xcode. DerivedData is rebuilt on the next build, so it’s safe to remove. xcrun simctl delete unavailable removes simulators for runtimes you no longer have.
• Docker. Clear it with Docker’s own tools, for example docker system prune, not by deleting its disk image, which throws away every image and container.
• Device backups. In Finder, select your iPhone or iPad, choose Manage Backups and delete the ones you no longer need.
• Caches of apps you don’t use anymore. Caches of apps you still use come back as you use them, so clearing those gives space back only for a while.

How DiskApp helps

DiskApp scans your Home folder or the whole disk and shows each folder by what it really holds, in a sunburst you can click into. It names the things above when it finds them: caches, idle build folders like node_modules (only when a project file proves it’s a project), Xcode and simulator data, AI models, Docker, deleted apps’ leftovers, and local snapshots.

Scanning is free. Clearing caches, idle build folders and leftovers, which DiskApp moves to the Trash, is part of DiskApp Pro. Data that a tool manages itself, such as Docker, the pnpm store, simulator devices and Time Machine snapshots, is never deleted by DiskApp. It shows the size and the exact command to run yourself.