How-to

System Data on Mac taking up 100 GB: how to clear it

The grey System Data block in Storage settings is a catch-all: caches, logs, snapshots, backups, virtual disks and hidden folders. On a Mac used for development it can pass 100 GB. Here is how to see what is inside it and clear it.

1. See how big it is

Open System Settings > General > Storage. On older macOS, use Apple menu > About This Mac > Storage > Manage. The System Data row is the number to beat. After a big cleanup it can take a restart before the figure updates.

2. Find what is inside it

Ask the filesystem instead of guessing:

du -sh ~/Library/Application\ Support/* 2>/dev/null | sort -rh | head -15
du -sh ~/Library/Caches ~/Library/Logs ~/Library/Developer ~/Library/Containers
du -sh ~/Library/Application\ Support/MobileSync/Backup
tmutil listlocalsnapshots /

The usual suspects on a developer Mac:

~/Library/Developer/Xcode/DerivedData and ~/Library/Developer/CoreSimulator: Xcode builds and simulators.
~/Library/Containers/com.docker.docker/Data/vms/0/data/Docker.raw: Docker Desktop's disk image.
~/Library/Application Support/MobileSync/Backup: iPhone and iPad backups.
~/.ollama/models and ~/.cache/huggingface/hub: local AI models. Hidden home folders like these often land in System Data.
~/.claude/projects and ~/.codex/sessions: coding-agent session history.
Local Time Machine snapshots and app caches in ~/Library/Caches.

3. Clear it, safest first

# old local Time Machine snapshots (use a date from the list)
sudo tmutil deletelocalsnapshots 2026-10-30-120000

# Xcode build data and dead simulators (rebuilt on demand)
rm -rf ~/Library/Developer/Xcode/DerivedData
xcrun simctl delete unavailable

# Docker: stopped containers, dangling images, build cache
docker system prune

# AI models you no longer use
ollama rm <model>
huggingface-cli delete-cache

Empty the Trash and restart afterwards. iPhone backups can be removed from Finder, under the device, Manage Backups.

What is safe and what is not

Safe: DerivedData, unavailable simulators, Docker's unused images and build cache, model files you can download again, caches of apps you have quit. macOS also drops local snapshots on its own when it needs room.

Not safe: deleting Docker.raw by hand (every image, container and volume goes with it), emptying ~/Library/Containers or ~/Library/Application Support wholesale (that is app data), anything under /System or /private/var/vm, and iPhone backups you have no other copy of.

Or let Prune do it

Prune lists the developer causes of System Data by name, with sizes: Xcode, simulators, Docker, Ollama and Hugging Face models, agent history and caches. Your own files go to the Trash, not straight to oblivion. The free scan shows every file before you pay. Pro is $15 a year or $29 once ($19 until 20 December 2026).

FAQ

Why is System Data so big on my Mac?

macOS counts caches, logs, local Time Machine snapshots, iPhone backups, virtual machine and Docker disk images, simulators, and many hidden folders in your home directory as System Data. On a developer Mac, Docker, Xcode and AI model folders are the usual reasons it passes 100 GB.

Can I delete System Data directly?

No. It is not one folder. You clear it by removing the individual things inside it: old snapshots, Xcode build data, Docker images, model files, backups. The Storage bar then shrinks, sometimes only after a restart.

Does restarting my Mac clear System Data?

A restart clears some temporary files and swap, and makes macOS recalculate the number, so the bar can drop a little. It will not touch snapshots, backups, Docker or model files.

Can Prune clear System Data?

Prune finds the developer-side causes by name: Xcode DerivedData and simulators, Docker, Ollama and Hugging Face models, coding-agent history and caches. You review every path first, and your own files go to the Trash.