System Data

One of the largest "contributors" to System Data are Time Machine (TM) local snapshots. Each snapshot is a copy of the volume being backed up. TM automatically saves one snapshot every hour, and keeps it for up to 24 hours.

To get a list of current snapshots, enter the following command in the Terminal app: sudo tmutil listlocalsnapshots /

The simplest way to delete these local snapshots would be to go to System Preferences > Time Machine. Then uncheck the "Back Up Automatically" option. This may take awhile, but after checking with the listlocalsnapshots command, once they are deleted, be sure to re-enable the option if you wish to have TM make automatic backups.

Ref:

• What is “Other” storage on a Mac, and how… - Apple Community
• About Time Machine local snapshots - Apple Support

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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 addition 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.

Nothing is " Lost ".

It is just using great amounts of Space on the drive.

As both @ Tesserax and P. Phillips ( self ) have provided ideas and methods to investigate what exactly is using the space on the drive - it now becomes the User ( you ) who needs to understand and take action to possibly make space on the drive.