What is System Data on a Mac? What a developer Mac keeps there and what is safe to remove

2026-11-01 · about 5 min · by the Latent team

Open System Settings › General › Storage on a developer's Mac and the biggest bar is often a grey one called System Data, sometimes 100 GB or more, with no way to open it. It is not one thing. It is everything macOS could not file under a named category, and on a machine that runs Docker, Xcode and local models, most of it belongs to those tools.

Why the Storage panel cannot tell you more

The Storage panel sorts files by kind: Applications, Documents, Photos, iCloud Drive, Mail, macOS, and a Developer item for some Xcode data. A 40 GB Docker disk image is not a document, an Ollama model is not an app, and a folder called ~/.cache is invisible in Finder. macOS has no category for them, so they all land in System Data, next to its own caches and swap.

That is why cleaning tips written for ordinary Macs (clear browser caches, delete old iPhone backups) barely move the number on a developer machine. The weight is in places those tips never mention.

Find out what is inside yours

Three commands cover most developer Macs. The first lists the heaviest hidden and library folders in your home directory:

# Heaviest hidden folders and Library subfolders in your home
du -sh ~/.[!.]* ~/Library/* 2>/dev/null | sort -h | tail -20

# Inside Docker Desktop's disk image
docker system df

# Simulator runtimes installed by Xcode (each one is several GB)
xcrun simctl runtime list

Terminal needs Full Disk Access (System Settings › Privacy & Security) to read some ~/Library folders; without it du prints permission errors and undercounts.

What a developer Mac usually keeps in System Data

The marks follow the same rule as our other guides: R comes back on its own (rebuilt or re-downloaded), I is in your hands because it may be the only copy, P belongs to macOS and should not be touched.

Remove each part the way its tool expects

Deleting these folders in Finder works for some and breaks others. Model stores share files between models and keep an index; Docker keeps everything inside one disk image. Use the owner's command:

# Docker: unused images, containers and build cache
docker system prune -a
docker volume ls                 # look before you prune volumes; databases live here

# Ollama
ollama list
ollama rm <model>

# Hugging Face (newer CLI; older versions: huggingface-cli delete-cache)
hf cache ls
hf cache rm model/<org>/<name> --dry-run   # then without --dry-run

# Xcode simulator runtimes and devices
xcrun simctl runtime list
# preview runtimes unused for 90 days, or delete one by its identifier
xcrun simctl runtime delete --notUsedSinceDays 90 --dry-run
xcrun simctl delete unavailable

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

LM Studio models are removed from its My Models view. Simulator runtimes can also be removed in Xcode's settings under Components (Platforms in older versions).

Why the number does not drop right away

• The Trash. Files moved to the Trash still count until you empty it.
• Local snapshots. If Time Machine is on, a local snapshot can keep a deleted file's space in use until macOS removes that snapshot. tmutil listlocalsnapshots / shows them; macOS thins them on its own when the disk gets full.
• Docker's disk image. Pruning frees space inside Docker.raw; recent Docker Desktop versions give it back to macOS on their own, older ones need the disk limit lowered in Settings › Resources.
• The panel itself. Storage recalculates slowly. Close System Settings and open it again after a few minutes.