System Data is 251 GB!! Why so HUGE??

Unfortunately even those recommended apps may no longer be able to display the information for all areas even when launching the app from the command line with root permissions. I know on my Mac, OmniDiskSweeper will show a " + " next to some folders which indicates the app is unable to access some items within the folder (happens when I launch it with root privileges and it is mentioned in the OmniDiskSweeper documentation).

In addition, some storage space is completely hidden from macOS in the form of APFS snapshots which are typically created by backup software such as Time Machine. Even third party backup software will utilize APFS snapshots. You can use the instructions in the following Apple article to view APFS snapshots, but make sure to click on the "Data" volume on the left pane of Disk Utility, otherwise you may not get the option "Show APFS Snapshots" on the "View" menu found on the menu bar. Normally APFS snapshots will be automatically deleted in a day to a week, but some may stick around longer depending on the app which made it and the app's configuration settings.

View APFS snapshots in Disk Utility on Mac - Apple Support

Read this User tip, choose one of the recommended Utilities, download and run. In about ten minutes, it will tell you where the big Boat Anchor-sized items are:

What is “Other” storage on a Mac, and how… - Apple Community

Disclaimer: I am not running the latest OS so I cannot test anything on my machine, but here are recommendations found on these forums:

When you are reading the following, note that you have absolutely no control over that category; only the OS does:

See used and available storage space on your Mac – Apple Support (UK)

Snapshots are one of the few things you have control over:

https://derflounder.wordpress.com/2018/04/07/reclaiming-drive-space-by-thinning-apple-file-system-snapshot-backups/

Third, another recommendation is to erase and install - a pain to do but in severe cases, I'd say worth a try. Since that category changes space requirements dynamically, a simple restart or shut down/boot up may also help to clear out stuff.

PistolPete001 wrote:

I am afraid to delete something here - as that might be important data:

Your fears are well founded. That drive is where all USER data are stored. Do not touch with Disk Utility, or you will lose everything.