You open System Settings > General > Storage after upgrading to macOS 27 Golden Gate, and the biggest block on the bar isn't Photos or Applications. It's a gray segment called System Data, and it's enormous. In the first week, MacRumors forum threads have reported System Data passing 200 GB on one Mac and more than a terabyte on a Mac Studio.

System Data isn't one thing. It's everything macOS can't neatly file into another category. That makes the label useless for diagnosis, but it also means the space is almost always explainable. Here's how to find out what's in yours.

First: give it a week

Some System Data growth after a major upgrade is temporary and clears on its own:

  • Snapshots created during the upgrade.
  • Installer leftovers that macOS cleans up after a successful update.
  • Caches from the old system that Golden Gate no longer uses.
  • Indexing in progress. Golden Gate builds a new semantic index for Spotlight and Siri, which converts your files and their contents into vectors. Mail rebuilds its index too.
  • Model downloads. Apple Intelligence models arrive after the upgrade, and on Golden Gate users report 14 GB or more.

If System Data is large two days after upgrading, check again after a week of normal use with the Mac plugged in overnight. If it's still large, work through the steps below.

Step 1: Get honest numbers

Storage settings are slow to recalculate and include purgeable space in "available." Terminal gives you the truth.

Real free space:

df -h /System/Volumes/Data

Give Terminal Full Disk Access so the next commands can see everything: System Settings > Privacy & Security > Full Disk Access, add Terminal, then quit and reopen it.

Top-level breakdown of the data volume:

sudo du -xhd 1 /System/Volumes/Data 2>/dev/null | sort -h | tail -15

This lists the largest top-level folders. Expect Users and Applications near the top. If Library, private, or a hidden folder is unexpectedly large, you've found your direction.

Your own Library, which is counted in System Data:

du -hd 1 ~/Library 2>/dev/null | sort -h | tail -15

Step 2: Check the usual suspects, largest first

Local Time Machine snapshots

The most common reason for huge, unexplained System Data. Snapshots are stored on your startup drive and hold files that have since changed or been deleted. 

tmutil listlocalsnapshots /

If there are many, and your external Time Machine backup is current, ask macOS to thin them: 

tmutil thinlocalsnapshots / 999999999999 4

Leftover macOS installers and beta updates

If you ran the Golden Gate beta at any point since June, macOS staged installers for multiple builds, and they don't always clean themselves up. Each full installer is around 18 GB. 

sudo du -sh /Library/Updates 2>/dev/null
ls -lh /Applications | grep -i "install macos"

Delete old installer apps from Applications. If /Library/Updates is still large after you've finished upgrading and restarted, those staged files are no longer needed, but the folder is protected by the system, so don't force your way in. Restart, open Software Update and let it check once more, which often clears stale downloads. If it doesn't, turn Beta Updates off and check again.

Swap files

ls -lh /System/Volumes/VM

Swap grows when memory is under pressure and shrinks after a restart. Several gigabytes of swap files are normal on 8 GB Macs. Don't delete them; restart instead, and look at what's using memory.

iOS simulators and Xcode data

If you've ever installed Xcode, this is often the jackpot. Simulator runtimes alone can take tens of gigabytes. 

du -sh ~/Library/Developer 2>/dev/null
xcrun simctl runtime list
xcrun simctl delete unavailable

Remove simulator runtimes you don't use with xcrun simctl runtime delete followed by the runtime identifier. Xcode's DerivedData folder, ~/Library/Developer/Xcode/DerivedData, is a build cache and safe to delete.

Virtual machines and containers

Docker Desktop, Parallels, UTM, and VMware store large disk images, often inside ~/Library, where Storage settings count them as System Data. 

du -sh ~/Library/Containers/com.docker.docker 2>/dev/null
du -sh ~/Parallels ~/Library/Containers/com.utmapp.UTM 2>/dev/null

For Docker, docker system df shows what's using space and docker system prune removes unused data after confirmation.

App caches and support files

du -hd 1 ~/Library/Caches 2>/dev/null | sort -h | tail -10
du -hd 1 ~/Library/Application\ Support 2>/dev/null | sort -h | tail -10

Look for apps you've deleted but whose support folders remain, and for runaway caches from browsers, chat apps, music streaming apps, and creative tools. Clear large caches from inside the app when it offers the option; delete support folders only for apps you no longer have.

The Spotlight index

sudo du -sh /System/Volumes/Data/.Spotlight-V100 2>/dev/null

Golden Gate's semantic index is new, and its size depends on how much content you have. If it's unusually large and Spotlight has been indexing for more than a week without settling, rebuild it once with sudo mdutil -E /. Never delete this folder manually.

What not to do

  • Don't delete anything in `/System`, `/private/var/folders`, or `/private/var/db`. These hold live system data. Removing files there can break apps, updates, or startup.
  • Don't delete swap files while the Mac is running.
  • Don't remove the Spotlight folder by hand. Use mdutil.
  • Don't run cleaner apps that "purge System Data" in one click without knowing what they delete. The safe parts are the ones listed above, and you can remove them yourself.

When System Data is still huge

If you've checked everything above and the numbers still don't add up:

  1. Restart. Swap, some caches, and purgeable space are released.
  2. Boot into Safe Mode once, which clears certain system caches, then restart normally.
  3. Check other user accounts. Their data can appear as System Data from your account.
  4. Compare `df` with Storage settings. If Terminal shows plenty of free space, Storage settings may simply be stale. It can take a long time to recalculate after big changes.
  5. Report it. If System Data keeps growing day after day with no identifiable folder, send Apple a report through Feedback Assistant with the output of the du commands above.

The short version

  • System Data is a catch-all label, not a single thing. Measure it in Terminal.
  • Wait about a week after upgrading. Snapshots, indexing, and model downloads settle.
  • The biggest real culprits: local snapshots, leftover beta installers, simulators, VM and container images, and app caches.
  • Never delete files from system folders by hand. Everything worth removing has a safe way to remove it.