Where you see it
Open System Settings, choose General, then Storage. The colored bar splits your disk into categories such as Applications, Documents and Photos. Whatever macOS cannot put into one of those lands in System Data. On older versions of macOS the same category was called Other.
What is inside
System Data is not one thing. It is everything that does not belong to another category, and most of it falls into these groups:
- Caches. Files apps keep to work faster, in
~/Library/Cachesand elsewhere. They come back when needed. - Time Machine local snapshots. Hourly snapshots your Mac keeps until it can reach your backup disk. macOS removes them when it needs the space.
- Virtual memory and the sleep image. Swap files and the file that saves memory during sleep. A restart resets the swap files.
- Logs and temporary files. Usually small, occasionally not, when an app writes logs in a loop.
- App support data. Databases, models, game assets and other app data that live in Library folders.
What you can safely do
- Restart your Mac. It clears swap files and many temporary files. It is the cheapest fix there is.
- Check local snapshots. In Terminal, this lists them. macOS deletes them
on its own when space runs low, so this is mostly for understanding, not
for urgent action.
$ tmutil listlocalsnapshots / - Clear developer leftovers. If you use Xcode, derived data and old simulators can be among the largest folders on your disk. Storage settings usually lists them under their own Developer category rather than System Data. Derived data rebuilds on its own.
- Look before you delete. Find which Library folders are really large before removing anything, and prefer the Trash over permanent deletion.
Why the number keeps changing
Snapshots appear and disappear, caches grow during the day, and swap grows when memory is tight. A System Data figure that moves by tens of gigabytes is normal. It is worth acting on only when it stays large after a restart and a completed Time Machine backup.
