Almost 90 GB of unexplained "System Data"?

Near the bottom of the following article you’ll see System Data and it’s explanation.

See used and available storage space on your Mac - Apple Support

And you can use these articles to try to clear up some space:

Free up storage space on your Mac - Apple Support

Find and delete files on your Mac - Apple Support

Optimize storage space on your Mac - Apple Support

Manage Storage

Rebuild the Spotlight index on your Mac

What is “Other” storage on a Mac, and how can I clean it out?

OmniDiskSweeper Safe to use

GrandPerspective 

How to delete Time Machine snapshots on your Mac

See used and available storage space 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 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.

A couple of things to try first.

• > System Preferences > Time Machine now turn off (uncheck) Back Up Automatically. Wait while it deletes Local Snapshots then turn it back on again. Check to see if the System Data has decreased.
• Restart in Safe Mode, this will undertake various system maintenance tasks which may release space. Do a Normal Restart and check System Data