ScrubsWhere 543 GB of System Data went
We measured one working Mac on macOS 27, used daily for iOS, Android and web development, with du, tmutil and Storage settings on 2 November 2026. Storage settings put 542.8 GB in System Data. This is what we found behind it. It was a developer's Mac, so developer data shows up more than it would on most; on a typical Mac the usual parts are caches, app data, Time Machine snapshots and other accounts' files.
| Size | What | Safe to clear? |
|---|---|---|
| about 378 GB | Folders the account can't openThe data volume held 887 GB, but the signed-in account could read only 509 GB of it. Most of the rest is a second user account's home folder: about 345 GB when we last measured it as administrator, in October. Storage settings listed "Other Users & Shared" at just 17 MB, so another account's files most likely land in System Data. | That account's owner decides. Sign in to it, or remove it in Users & Groups if nobody needs it |
| 32.3 GB | Your caches ~/Library/CachesJetBrains IDE indexes 16.3 GB, Google's apps 4.9 GB, Spotify 4.2 GB, SwiftPM 2.3 GB, and dozens of smaller ones. | Yes; apps rebuild them |
| 31.6 GB | App support data ~/Library/Application SupportApps' own data that isn't a document: a desktop AI app 14.7 GB, JetBrains IDEs 8.5 GB, Notion 3.7 GB, VS Code 1.5 GB. | Only through each app, or for apps you've deleted |
| 18.7 GB | Temporary files /private/var/foldersPer-user temporary and cache folders macOS and apps write. Most clears on restart or by itself. | Restart; don't delete by hand |
| 12.2 GB | Sandboxed apps' data ~/Library/ContainersDocker Desktop was all of it here; App Store apps keep their data here too. | Through the app; see Docker.raw |
| 9.8 GB | Apple's downloads /System/Library/AssetsV2Siri and dictation models, voices, dictionaries, developer documentation. The same folder held 28.9 GB of simulator runtimes. | No, except simulator runtimes through Xcode |
| 8.5 GB | Swap and sleep image /System/Volumes/VM, /private/var/vmSix 1 GB swap files at the time, on their own VM volume, and a 2.1 GB sleep image. | Restart to shrink swap |
| 6.1 GB | Simulator caches and logs /Library/Developer/CoreSimulator/Caches, ~/Library/Logs, /Library/Logs3.8 GB of simulator caches, 2.3 GB of logs. | Yes |
| 0 GB | Time Machine local snapshotstmutil listlocalsnapshots / listed none on this Mac. On a Mac with Time Machine on, they can be tens of GB. | macOS removes them when space runs low |
The rest was developer data Storage settings didn't count as Developer: it put 53.5 GB there, but ~/Library/Developer alone held 83 GB, with iOS DeviceSupport, DerivedData and simulator devices. Counts overlap a little, because du counts a cloned file once per copy.
The lesson: on a Mac with more than one account, check the other accounts before anything else. On this one, they were two thirds of System Data, and no cleanup in the signed-in account would have touched them.
What System Data is
System Settings › General › Storage sorts your disk into categories such as Applications, Documents, Photos and Developer. Everything macOS can't put in one of those lands in System Data, which was called "Other" before macOS Monterey. It isn't one folder: it is whatever is left over, spread across your disk.
It often includes:
~/Library/Caches and /Library/Caches./System/Volumes/VM and the sleep image in /private/var/vm./private/var/folders, Spotlight's index, and files in folders macOS hides.Part of it may be purgeable: space macOS can free on its own when an app needs it. That is why the bar sometimes shrinks without you doing anything.
Why System Data is so big, or keeps growing
System Data grows when something writes where Storage settings has no category for it. On the Mac we measured, these were the causes:
~/Library/Caches as you use them. JetBrains IDEs alone kept 16.3 GB of indexes.~/Library/Application Support or ~/Library/Containers: 14.7 GB for one desktop AI app, 12.2 GB for Docker.How to see what is in it
Click the ⓘ next to a category in System Settings › General › Storage for macOS's own suggestions. For System Data itself there is little detail.
List the biggest folders in your Library, smallest first:
du -hd 1 ~/Library 2>/dev/null | sort -h
List Time Machine's local snapshots on the startup disk:
tmutil listlocalsnapshots /
Terminal needs Full Disk Access in System Settings › Privacy & Security to read some folders, and du reports what it can't open as errors.
A map shows the whole disk at once, so the big folders stand out without guessing where to look. In Scrubs, scan Macintosh HD with Full Disk Access on, then follow the largest rectangles. Two special tiles explain the rest:
macOS and system volumes: the read-only system and the volumes macOS keeps beside your data, such as Preboot, Recovery and VM.
Unaccounted: space the scan could not open, such as other users' folders. Scan as administrator to see it.
How to clear System Data: what is usually safe
- Your caches in
~/Library/Caches. Apps rebuild them, and some open slower the first time after. - Local snapshots. macOS removes them when space runs low. To remove one now, use the date that
tmutil listlocalsnapshots /shows:tmutil deletelocalsnapshots 2026-10-12-101500. - Old iPhone and iPad backups. Finder › your device › Manage Backups.
- Mail's attachment copies in
~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads. Mail saves one each time you open an attachment; the message keeps the original. - Data of apps you've deleted in
~/Library/Application Supportand~/Library/Containers. Uninstalling an app by dragging it to the Trash leaves its data behind.
If you develop software
- Xcode's DerivedData in
~/Library/Developer/Xcode/DerivedData. Xcode rebuilds it on the next build. - Old simulator runtimes. Remove the ones you don't need in Xcode › Settings › Components. Simulators for runtimes you no longer have can go with
xcrun simctl delete unavailable. - Device support files in
~/Library/Developer/Xcode/iOS DeviceSupport, for iOS versions you no longer debug. Xcode copies them again when a device on that version connects. - Old
node_modulesin projects you aren't working on.npm installbrings them back. - Docker. Remove unused images and volumes with Docker's own clean-up, or lower its disk limit in its settings. Its disk image does not always shrink by deleting containers.
Move to the Trash, don't delete for good. If something breaks, you can put it back. Empty the Trash once everything still works.
What to leave alone
- Anything in
/System. It is read-only, and macOS manages it. - The swap files and sleep image in
/System/Volumes/VMand/private/var/vm. Restarting usually shrinks swap. - Folders inside
~/Library/Containersand~/Library/Group Containersyou don't recognise. They hold apps' data, sometimes your documents. ~/Library/Mobile Documents, which is iCloud Drive, and~/Library/Mail.
When you don't know what something is, look it up before deleting it.
For everything outside System Data, see how to free up disk space on a Mac, and the Mac folder guide for what each big folder is.
Questions
Can I delete System Data on a Mac?
Not as a whole: it isn't one folder. You can clear parts of it, such as your caches, old iPhone backups, Mail's attachment copies and Time Machine's local snapshots. Leave /System and folders you don't recognise alone.
Why does System Data keep growing?
Apps keep adding caches and data, Time Machine keeps local snapshots, and macOS adds swap when memory runs short. Much of it shrinks again on its own; a restart clears swap and temporary files.
Why is System Data so big on my Mac?
Most often another user account's files, large app data such as virtual machines or downloaded models, caches, and local snapshots. On the Mac we measured, another account was two thirds of it.
Is it safe to clear System Data in Terminal?
Commands like rm skip the Trash, so a mistake can't be undone. Move things to the Trash in Finder instead, and empty it once everything still works.
What was System Data called before?
Other, until macOS Monterey.
How Scrubs helps
Scrubs maps the whole disk, so System Data stops being one grey bar. Its Quick Wins list the safe-to-clear folders above with their sizes, and iPhone backups and old downloads under Worth a look, and everything goes to the Trash first, with ⌘Z to undo. Scanning is free.
Written October 2026, for macOS 26 and 27.
About the author: Parsa Nasirimehr is a mobile app developer who builds Scrubs, a disk map for the Mac. Spotted something wrong? Write to support@getscrubs.app.