Updating to macOS 27 Golden Gate? Check Your "System Data" First

The Mac subreddits are full of the same post this week. Someone updates to macOS 27 Golden Gate, opens Storage settings, and finds "System Data" at 150 GB. Deleting documents doesn't move the bar. Sometimes it grows back after they've deleted 100 GB.

I haven't updated yet. I'm still on macOS 26 Tahoe, and my System Data was already 147 GB of a 245 GB disk. So before installing Golden Gate, I split that grey bar into its parts.

Why deleting documents doesn't help

Storage settings only names a few things: Apps, Documents, Photos, Mail, iCloud. Everything else lands in System Data: app data, caches, backups, snapshots, swap. So the "Documents" number is small, and the grey bar is where the space actually went.

What was inside mine

• App Support Data, 37.55 GB. Folders apps keep in ~/Library/Application Support. On my Mac it's mostly AI tools; the Claude desktop app alone is 13 GB.
• Shared Library & Homebrew, 31.92 GB. Developer tools in /opt/homebrew and shared app data.
• Virtual memory (swap), 20.44 GB. Memory macOS parked on disk. A restart clears most of it.
• Other Library data, 13.54 GB, and App Containers, 4.9 GB. More per-app data.
• Not measurable, 11.21 GB. Parts macOS won't let any app measure directly.
• System caches and temp files, 10.62 GB, plus Caches, 4.77 GB. Rebuilt on their own.
• Developer and AI caches, 8.43 GB. Package and model caches.
• System assets and AI models, 3.83 GB. Apple Intelligence and other downloaded system parts.

Why it jumps after a macOS update

• Time Machine local snapshots. macOS keeps snapshots on your own disk. Files you delete stay inside them, which is why space doesn't come back after a big cleanup.
• Re-indexing. After an update, Spotlight and Photos re-scan everything. That fills caches and swap for a day or two.
• Apple Intelligence. Its models download after the update and count as system storage.
• The installer itself. A major update needs roughly 20–40 GB free. Near full, it fails or the Mac crawls.

What to do before updating (10 minutes)

1. Restart. That alone clears most swap.
2. Check snapshots in Terminal. If the first command lists any, the second removes them. Backups on your external drive aren't touched.

tmutil listlocalsnapshots /
sudo tmutil thinlocalsnapshots / 999999999999 4

1. Open Finder › your iPhone › Manage Backups and delete backups of old devices.
2. See your biggest app folders:

du -sh ~/Library/Application\ Support/* 2>/dev/null | sort -h | tail

1. If you use Homebrew: brew cleanup --prune=all
2. Start the update with at least 30–40 GB free.

What not to delete

Anything in /System, your ~/Library/Preferences, and the folders of apps you still use. Manage Photos, Mail and Messages from inside those apps. And never "clean" the whole Library folder: move things to the Trash one at a time, so a mistake costs nothing.

Full disclosure: I build Tidy Mac, the app in the screenshot. It splits System Data into named parts like these and moves what you pick to the Trash with Undo. You don't need it for the steps above. If you just want to know whether one folder is safe to delete, there's a free checker, and a longer guide to System Data on macOS 27.

Drafted with AI assistance. Every number is from my own Mac.