What is “System Data” on a Mac, and can you delete it?

Open System Settings › General › Storage and you may see a huge grey bar called System Data, sometimes tens or hundreds of gigabytes. It looks mysterious, and Apple doesn’t explain it. Here is what it really is.

What “System Data” contains

It is a catch-all category: everything macOS cannot file under Apps, Documents or Photos. That typically includes:

• Caches that apps and the system rebuild when needed.
• Logs and diagnostic reports.
• Local Time Machine snapshots and other APFS snapshots.
• Application support files, plugins and extensions.
• Virtual memory (swap) and sleep images.
• Spotlight indexes and other system databases.
• Often, old iPhone backups and developer files like Xcode’s build folders and simulators.

Can you delete it?

Not as a whole. A large part belongs to macOS and it manages it for you; deleting system files by hand can break things. But some of what sits inside is yours to clean safely:

• Old app caches that regenerate on their own.
• Developer leftovers such as Xcode DerivedData, unused simulators and package caches.
• Old iPhone backups you no longer need (check first).
• Installers (.dmg, .pkg) you already used.

Local snapshots disappear by themselves, usually within 24 hours or when space runs low.

How to see what is inside

Undig’s X-Ray splits your disk into understandable parts: your files, your personal Library, apps, the sealed system volume, hidden APFS volumes, snapshots and the Trash. For each cleanable item, it explains what it is and marks it Safe or Review. Everything goes to the Trash first and is recorded in a Journal, so you can undo it.

If you deleted files and your free space did not change, read why disk space isn’t freed after deleting.