System Data is taking up so much space on my Mac mini

This issue appears every day here in the Community.

Rule 1 of using Macs is "Don't install junk."

Keep your software updated and macOS is capable of keeping itself secure. MacOS can take care of itself without the need for useless cleaners, optimizers and anti-virus apps.

Clean My Mac is considered by many veteran users to be nothing more than snake oil and likely to cause more problems than it prevents. I've been using well performing Macs for decades without these types of apps. I'd uninstall CMM.

Meanwhile, What you can do...

• Empty the Trash in the Dock.

• Empty the trash in the Photos app.

• Delete unused and unneeded installers from your Downloads folder and desktop.

• Reboot your Mac at least weekly.

• Transfer files that you don’t use daily to an external drive and then delete them from the startup drive and empty the trash. Files that take up the most room are movies, images and music.

• Force Time Machine to delete local snapshots:

About Time Machine local snapshots - Apple Support

• Use Disk Utility to delete APFS snapshots: View APFS snapshots in Disk Utility on Mac - Apple Support 

This from Apple on the subject of storage management:

Free up storage space on Mac - Apple Support

daltons63 wrote:

System data takes almost 400 GB of space.

this doesn't seem to be normal tot me

any tips ? help to reduce this ?

clean mymac alleready cleaned things that could be cleaned but this changes not a lot

I have never had trouble with too large System data so I wonder why some people have issues with it. Some possible explanations:

• I never look at the System data because it is unreliable. Instead, I look what Finder reports in its folders' Status Bar (View > Show Status Bar) or what Disk Utility reports as available or free space.
• I never use any 3rd party virus apps (Avast, AVG, Bitdefender, Intego, MacAfee, Norton, Kaspersky, ESET NOD32, Trusteer, PC Matic etc) and 3rd party "cleaning", "optimizing", "speed-up" apps (CleanMyMac, Cleaner One, MacKeeper etc). They produce obscure errors and corruption and are not needed. Sometimes cleanly reinstalling macOS and restoring only user data files is needed to fix the damage they have done.
• I use Carbon Copy Cloner for backups. I don't know if Time Machine might sometimes create extra snapshots that take too much space. But macOS should automatically delete such snapshots after a few days anyway. If not, the user should be able do that manually via Disk Utility.