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

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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.

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