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What is System Data on a Mac, and how do you shrink it?

Open System Settings › General › Storage and you’ll often see a large grey bar called System Data. It can grow to 50 GB or more, and macOS doesn’t say what’s in it. That’s because it isn’t one thing: it’s everything macOS doesn’t sort into Applications, Documents, Photos and the other categories.

What’s usually inside

  • Caches that apps and macOS keep in Library/Caches folders
  • Logs and diagnostic reports
  • Local Time Machine snapshots, if you back up with Time Machine
  • Virtual memory swap files and the sleep image
  • Developer data such as Xcode builds, simulators and package-manager caches
  • Disk images of virtual machines and containers, such as Docker’s
  • Data apps keep in Application Support and Containers folders

Most of it lives in Library folders that Finder hides by default, which is why it’s hard to track down.

Start with the quick wins

  • Restart your Mac. Swap files and many temporary files are cleared when it starts up again.
  • Empty the Trash.
  • If you use Time Machine, check for local snapshots. macOS removes them by itself when space runs low, but until then they can hold tens of gigabytes.

Remove local Time Machine snapshots

To list them, open Terminal and run:

tmutil listlocalsnapshots /

Each snapshot’s name ends with a date, such as 2026-10-29-120000. To delete one, run this with its date. macOS asks for your password.

sudo tmutil deletelocalsnapshots 2026-10-29-120000

Your backups on the Time Machine drive aren’t affected. Only the copies kept on your Mac are removed.

Find the big folders with a disk map

Storage settings won’t show the folders inside System Data, but a disk map will. Give OpenDisk Full Disk Access, scan your startup disk, and the largest folders appear as the widest slices. The space is usually in a few places:

  • ~/Library/Caches and /Library/Caches
  • ~/Library/Developer, for Xcode builds, simulators and archives
  • ~/Library/Containers and ~/Library/Group Containers
  • ~/Library/Application Support

OpenDisk also lists common developer caches, such as Homebrew, npm and Xcode DerivedData, so you can see which ones are worth clearing.

What not to delete

  • Anything in /System or /private/var/vm. macOS manages these, and the system volume is read-only anyway.
  • An app’s whole Application Support or Containers folder while you still use the app. That’s usually your data, not a cache.
  • Caches are safe to delete, but apps rebuild them as you use them, so some of that space comes back.

When in doubt, move things to the Trash instead of deleting them permanently, use your Mac for a day, then empty it.