His Mac Cleaner Freed 3 GB. The Other 40 GB Was Hiding in “System Data”.

A developer friend's MacBook Air kept saying the disk was almost full. He was already paying for CleanMyMac. He ran its clean-up, and it freed about 3 GB.

Then he looked inside the grey bar every Mac owner has learned to hate: System Data. About 40 GB came back from inside it.

The cleaner wasn't broken. It was looking in the wrong places. On a developer's Mac, most of the space isn't caches or logs. It's build files, simulators, package caches and old installers, all filed under System Data.

Check your own Mac in one command

It only measures; it deletes nothing.

du -sh ~/Library/Developer ~/Library/Caches ~/.npm ~/Library/Containers/com.docker.docker ~/Library/Application\ Support/MobileSync/Backup 2>/dev/null

A missing line means the folder doesn't exist (or Terminal needs Full Disk Access). Anything over 5 GB is worth a look.

1. Xcode build files

~/Library/Developer/Xcode/DerivedData rebuilds itself on the next build. Quit Xcode, then empty it.

2. Simulators and device support

xcrun simctl delete unavailable

• Remove old simulator runtimes in Xcode → Settings → Components (Platforms in older Xcode).
• ~/Library/Developer/Xcode/iOS DeviceSupport keeps a folder per iOS version per device. Delete the old ones; Xcode copies them again when needed.

3. Package caches

npm cache clean --force
brew cleanup --prune=all
pip cache purge

Old node_modules in projects you no longer touch add up too. npm install brings them back.

4. Old installers

find ~/Downloads -maxdepth 1 \( -name "*.dmg" -o -name "*.pkg" \) -exec du -sh {} +

Plus any "Install macOS" app in Applications.

Not on his Mac, but check yours

• iPhone backups: Finder → your iPhone → Manage Backups.
• Docker: docker system df, then docker system prune for what's unused.
• Time Machine snapshots: tmutil listlocalsnapshots /. Leave them; macOS removes them when it needs space.

What not to delete

• Anything under /System, or whole folders in ~/Library. Delete specific items, never the parent.
• ~/Library/Mobile Documents (that's iCloud Drive).
• Anything inside the Photos library.

When in doubt, move it to the Trash and empty it a week later.

Is a cleaner worth paying for?

If you don't write code and your photos live in iCloud, probably not: turn on the recommendations in System Settings → General → Storage.

If System Data is huge and you'd rather not hunt folder by folder, an app that names every part of it saves the time.

Full disclosure: I build Tidy Mac, the app my friend used. The download is free and shows everything filling your Mac; cleaning past the first 500 MB is $9.99 once, for two Macs. Full case study: CleanMyMac freed 3 GB, Tidy Mac found 40 GB.

How big is your System Data? Tell me in the comments.

Written with help from an AI assistant. The 3 GB and 40 GB figures are from my friend's Mac.