Why your Mac's storage graph lies to you

Open System Settings → General → Storage on a Mac that's been in use for a year and you'll likely meet a bar segment labelled System Data — often larger than your apps and photos combined. Apple's documentation describes it vaguely, and for good reason: it's the drawer everything gets swept into.

#What's actually in there

"System Data" is a grab-bag of caches, logs, snapshots and support files, including:

• Developer caches — Xcode's DerivedData and old device support files
• Package manager downloads — Homebrew, pip, npm, Cargo
• App support files that stopped being useful long ago
• Time Machine local snapshots
• System logs and crash reports

None of it is "the system" in the sense that removing it breaks macOS. The label makes it look untouchable, so it quietly becomes the biggest thing on your disk.

#Time Machine local snapshots are the sneakiest

APFS keeps local snapshots so Time Machine can work while your backup drive is unplugged. They're supposed to self-expire after 24 hours — but space pressure, failed backups and plain bugs can leave several behind. Check yours:

tmutil listlocalsnapshots /

Each snapshot freezes every changed file on the disk, which is why deleted files sometimes "come back" as unexplained used space. Deleting a snapshot you don't need is safe:

sudo tmutil deletelocalsnapshots <timestamp>

#The 80/20 of reclaiming System Data

In practice, four moves recover almost everything worth recovering:

1. Clear Xcode's DerivedData if you've ever built an iOS or Mac app.
2. Run brew cleanup -s if you use Homebrew.
3. Delete unavailable simulator devices (xcrun simctl delete unavailable).
4. Prune stale Time Machine local snapshots.

Do those and most Macs give back 20–60 GB without touching a single document, photo or app.

MacVoid was built because those four moves shouldn't require a terminal. It scans exactly these locations, shows what it found, and waits for your approval before anything goes to the Trash.

#What not to chase

SIP-protected system volumes, /private/var/vm sleep images, and anything inside /System are managed by macOS. Tools that claim to "clean system junk" there are selling theater — the directories are either read-only or recreated instantly. If a cleaner's headline number comes from those places, it isn't a number you'll still have tomorrow.

#Keep going

For the complete playbook, read how to free up space on your Mac, or see which hidden caches are actually worth clearing if you develop on your Mac.