How do I reduce 411GB System Data usage on Mac?

Hello, Fred.

It sounds like your System Data is taking up a significant amount of storage, which can sometimes be caused by APFS snapshots or other system-related files. Here are a few things you can try:

1. Check for APFS snapshots: Open the Terminal and type tmutil listlocalsnapshots / to see if there are any snapshots. If you have snapshots, you can delete them by running sudo tmutil deletelocalsnapshots <snapshot-name>.
2. Reboot your Mac: Sometimes, a simple restart can help reset how storage is being reported.
3. Optimize Storage: Go to Apple Menu > About This Mac > Storage > Manage. Here, you can review options to optimize your storage by removing old files, unused apps, or large files that you no longer need.
4. Clear System Caches: Use a tool like CleanMyMac or manually clear cache files in Finder. Be cautious and avoid deleting system files without knowing what they are.
5. Check Time Machine: If you think Time Machine backups are the culprit, ensure that the Time Machine drive is connected and performing as expected. You can also try turning Time Machine off temporarily to see if that affects the storage usage.

Hopefully, one of these steps helps reduce the size of your System Data.

I hope this helps!