What’s in System Data on a Mac, and how to shrink it

What Apple says System Data is

Apple describes System Data (called Other in older versions of macOS) as a general category that measures the space used by Apple and third-party files that don’t belong to a more specific category. Apple’s Mac User Guide notes, “You can’t manage the contents of this category.” Apple doesn’t publish a full list of what’s inside, and it varies between macOS versions.

What usually ends up there

• Caches and logs in ~/Library/Caches, ~/Library/Logs and their system-wide equivalents.
• App support data in ~/Library/Application Support: databases, downloaded content, offline libraries, chat histories.
• Developer data such as Xcode’s DerivedData, simulators and device support files in ~/Library/Developer.
• Virtual machine and container images, such as Docker Desktop’s disk image, which can grow to tens of gigabytes.
• Local Time Machine snapshots, which macOS keeps while there’s room and removes when space is needed.
• Virtual memory and sleep files that macOS manages itself.

How to look inside

With a disk map

Scan your home folder, or just ~/Library, with DiskTree. Folders are drawn to scale and coloured by kind, so oversized caches, toolchains and app data stand out. Without Full Disk Access some protected folders are skipped and listed as unreadable; granting it is optional. Snapshots, swap and purgeable space aren’t ordinary folders, so no folder map, including DiskTree’s, can show them.

With Terminal

These commands only read. To list the biggest folders in your Library:

du -h -d 1 ~/Library 2>/dev/null | sort -h | tail -15

To list local Time Machine snapshots:

tmutil listlocalsnapshots /

Safe ways to shrink it

• Clear caches in the app that made them where it offers an option, for example a browser’s or a music app’s offline cache.
• Remove developer caches: Xcode’s DerivedData, unused simulators and old device support files. See the developer guide.
• Prune container images with Docker Desktop’s settings or docker system prune.
• Delete old iPhone and iPad backups from Finder: select the device, then Manage Backups.
• Empty the Trash, then restart. A restart lets macOS clear temporary and virtual-memory files.
• Give it time. Local snapshots and purgeable caches are released when macOS needs the space.

What not to do

• Don’t delete folders inside /System, /Library or /private. macOS needs them, and deleting there can stop apps or the system from working.
• Don’t run cleanup scripts or “cleaner” commands you don’t understand.
• Don’t clear every cache at once. Apps rebuild them, so the space often comes straight back.
• Back up before removing anything you can’t identify.

Sources

• Apple: Free up storage space on Mac
• Apple: Find and delete files on your Mac