SB-00 · Department
How macOS counts capacity before anyone deletes anything. Inventory pass completed October 20, 2026, carrying 4 worked fiches.
Every cleanup argument ends at the same place: a number somebody read somewhere. This department builds the reading habits first, checking capacity four ways, watching which categories swell without permission, and naming the anonymous System Data bucket so it stops intimidating.
Nothing here asks for software yet. The pieces are deliberately short on drama and long on measurements you can reproduce immediately, because a drive audited calmly stays healthy longer than one purged in panic. Other trades settle their arguments the same way: what counts as a bon site d'élevage gets decided against criteria written down in advance, not by whoever states an opinion loudest.
System Data will not open in Storage settings. How to look inside it with Terminal and Finder, which residents to check first, and what to leave alone.
Four ways to see how much room is left on a Mac drive, from About This Mac to df, and why two of them can disagree.
The System Data bucket gathers caches, logs, virtual memory and more; a plain tour of what lives inside.
Caches, snapshots, synced libraries and growing mail stores explain silent growth; here is how each behaves.
Reading order. Start with the measurement piece if you are pressed for time; every later guide assumes its numbers.