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

System Data is the bucket macOS uses for everything it could not classify. Here is what is usually inside it, and which parts are safe to clear.

· 7 min read · Shaheer Malik

If your storage bar shows 80 GB of System Data, nothing has gone wrong. That category is a remainder: it holds everything macOS did not sort into Applications, Documents, Photos, Mail or macOS itself. The useful question is which of those remainders is big on your Mac.

What is usually in it

  • Caches — application caches, browser caches, package manager caches. They rebuild.
  • Logs and diagnostic reports, in ~/Library/Logs and /Library/Logs.
  • Local Time Machine snapshots, which macOS keeps on your disk and thins automatically when space runs low.
  • iOS simulator runtimes and Xcode derived data, if you have ever installed Xcode.
  • Mail downloads and attachments.
  • Virtual machines, container images, and anything else stored inside an application’s container.

What is safe to clear, and what is not

Caches and logs are safe: an application that loses its cache rebuilds it, at the cost of one slower launch. Derived data and simulator runtimes are safe and easily the largest win on a developer’s Mac. Local snapshots are best left to macOS, which removes them as it needs the space. Anything inside a container that is not a cache is that application’s data — treat it as you would a document.

Do it in this order

  1. Empty the Trash. It counts against you until you do.
  2. Clear ~/Library/Caches and ~/Library/Logs, one folder per application so you can see what you are removing.
  3. If you have Xcode: remove ~/Library/Developer/Xcode/DerivedData and any simulator runtimes you no longer use.
  4. Look in ~/Library/Containers for applications you no longer run, and uninstall them properly rather than dragging the app alone to the Trash.
  5. Restart. Some space is only released when the processes holding those files let go.

Why the number does not drop straight away

macOS reports free space with a delay, and it counts purgeable space — local snapshots and caches it could remove if it had to — in ways that can look like nothing happened. Give it a few minutes and a restart before you decide a cleanup did not work.

DiskDuo shows these places as they are on your own disk, with what each one holds, and moves what you choose to the Trash. Nothing is removed without you seeing the list.

Download DiskDuo

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