What's usually inside System Data
- App caches in
~/Library/Caches, rebuilt automatically when needed - Old iPhone and iPad backups in
~/Library/Application Support/MobileSync - Xcode DerivedData, archives, device support files and iOS simulators
- Package manager downloads from Homebrew, npm, Cargo, pip and Gradle
- Logs, Time Machine local snapshots and leftovers from deleted apps
Clear it with Broom
- Scan with Full Disk AccessBroom needs it to measure Library folders that macOS protects.
- Open CleanupSafe to clear items, like caches and build folders, are already selected. Your own files never are.
- Review Worth a lookBackups, simulators and big old files are listed with sizes, for you to tick.
- Move to TrashEverything goes to the Trash with its original location recorded, so Put back works later.
Check Time Machine local snapshots
If System Data is huge right after a big delete, local snapshots are the usual reason. Time Machine keeps hourly snapshots on your startup disk for about 24 hours, and the space they hold doesn't come back until they expire. List them, then thin them if you need the space now:
tmutil listlocalsnapshots /tmutil thinlocalsnapshots / 50000000000 4The second command asks macOS to free up to 50 GB of snapshots at the highest urgency. It only removes local snapshots; backups on your Time Machine drive are untouched.
What not to delete by hand
Don't delete folders in /System, /private/var or your top-level folders. Broom checks every path before it moves and refuses those. Swap files in /private/var/vm and sleep images shrink on their own after a restart.
Questions
Why is System Data so big on my Mac?
Usually caches, iOS backups, Xcode files and simulators. They're counted as System Data because macOS doesn't know what else to call them.
Is it safe to clear System Data?
Caches and build folders are safe: apps rebuild them. Backups and simulators are safe to remove if you don't need them. System files should never be touched, and Broom won't let you.
Will clearing caches make my Mac faster?
Mostly it frees space. Apps rebuild their caches as you use them, so the first launch afterwards can be a little slower.
Why did System Data grow after I deleted files?
Local Time Machine snapshots still hold the deleted files until they expire, usually within 24 hours. Thinning them with tmutil frees the space right away.