System Data Taking up 128.18 GB

Probably the answer the user does not what to hears but here goes

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GrandPerspective 

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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.

System data consists of all sorts of files produced by various system processes.

Try the following.

• start up in safe mode followed by a normal restart
• force Time Machine to delete local snapshots by turning TM off in system preferences, waiting while TM deletes the local snapshots then turning it back on again.
• it is good practice to shut down the Mac on a regular basis.

Don't be tempted to install any clean-up apps.

I agree with Mal-S's suggestions. System Data (formerly 'Other') cannot be managed by the user directly.

This User Tip is a good read - What is "Other" and What Can I Do About I… - Apple Community