System Data - using over 100GB

If the user Wiped the Drive to Zero and reinstalled the Operating System as band new - this issue would not occur.

It would occur, if Time Machine Backup was used post Clean Installation and migrated everything into the computer.

Rebuild the Spotlight index on your Mac

Or Force a Reindex via Terminal Command “ sudo tmutil -E / “ without Quotation Marks 

OmniDiskSweeper Safe to use

GrandPerspective 

How to delete Time Machine snapshots on your Mac

Locate backups of your iPhone, iPad, and iPod touch

Notation - If the user is using a cloning software like Carbon Copy Cloner - suggest tweaking the Safety Net Feature in this software. It may be making additional Snap Shots that are not being Cloned to the Eternal Drive. If this should be the case, these Snap Shot could be using additional space on the drive 

The final word from Apple on Managing the " Other/ System Data “ Category

Other / System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

If you determine that the "System Data" category is occupying an inexplicably large amount of storage, refer to See used and available storage space on your Mac - Apple Support. Pay particular attention to the description of System Data.

You may find neuroanatomist's User Tip helpful: What is "Other" storage on a Mac, and how can I clean it out? - Apple Community.

• What Apple used to label "Other" is now labeled "System Data".

Generally, the files in the System storage can be anything that cannot be categorized into app, movie, picture, music or document, such as:

1. Essential macOS operating system files;

2. Time Machine snapshot backups;

3. System log files and cache;

4. Cache from Browsers, Mail, photos and the third-party apps;

5. Old macOS updates;

6. Trash data and junk files.

Sometimes your Mac builds up an unnecessary amount of System files due to a bug, but most of the time it’s a normal part of how the operating system works. However, you can follow the below methods to manually reduce system storage on your Mac:

1. Empty Trash

2. Manage Time Machine Backup Snapshots

3. Optimize your Storage

4. Remove iOS Backups

5. Delete Cache Files

6. Update macOS

Hope it helps!

Hello muzikT70,

Available information to optimize your Mac storage can be found here: Free up storage space on your Mac - Apple Support

We're unable to definitively isolate what is being reported as "System Data" ,however, if you believe it is related to apps such as Logic Pro you may be able to isolate by temporarily moving or removing related projects. If your reported usage varies you'll have a definitive answer.

Regards.