1 - This issue may or may not be Directly related to How Much Empty Space is available on the Drive
2 - It can also be directly related to the Software installed and Used on the computer
3 - It can be a combination of the two above
A1 - Rebuild the Spotlight index on your Mac
What is “Other” storage on a Mac, and how can I clean it out?
Free up storage space on your Mac
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 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.
A2 - Download the Application Etrecheck directly from the Developer.
This is a Diagnostic Tool that makes no changes to the computer.
It makes a coherent and readable inventory of both the Hardware and Software used on the computer
The application is free or paid from added features.
The Report will Not Reveal Any Personal Information.
Post back the Full Report - copy and paste - >>>> using the Additional Text Icon ( 3rd Icon to last ) <<<<
We can have a look at the report for possible issues and may have possible suggestions to resolve the issues
A3 - Any Third Party Applications that will interfere with the normal operation of the OS, is an invitation for disaster and comprising the Operating System
Certain Applications maybe available on the Apple Apps Store - this only means the Developer is prepared to pay Apple a portion on each sale. What the Application may do to the computer is up to the User to check this out before purchase
Any of the below should be removed as per Developers Instructions
This will include CleanMyMac , This will include BitDefender ,
This will included Norton Antivirus , Sophos Av Software
Intego AntiVirus, McAfee, MacKeeper, Avast AntiVirus
Ad Guard, Webroot , ESET , Kaspersky Antivirus. Anti virus
AVG AntiVirus PowerMyMac
Trustee Total AV avira antivirus Dr Cleaner Pro Plus
AntiVirus for Mac
F-Secure
Read some of the posting and arrive at your own conclusions.
The The Built in Security is all that is required.