Storage issue

Manage Storage

It is generally a good computer practice to alway keep at least 15% to 20% of the Total Drive Capacity’s as Empty Space.

Allowing the computer to drop below these guidelines may eventually, cause unintended consequences including the inability to upGrade to a new version of macOS.

Starting in macOS 10.15 Catalina - the Operating System ( Catalina ) resides in a READ ONLY Volume and can not be opened and modified by the User.

That is by Apple Design to protect the Operating System from users mucking around with the System

The links below will assist in identifying what is taking up space on the Internal Drive and provide possible ways to remove data that is under the direct control of the User ( Home Folder ) . 

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.

Hello med_ah,

Thanks for posting in the Apple Support Communities regarding your Mac storage.

To free up space on Mac, we recommend following the steps below to ensure nothing that is important for the Mac to run gets deleted.

"Optimize storage space

Your Mac can optimize storage by using iCloud to automatically make more storage space available when needed.* And you can use built-in utilities to quickly find and delete files, apps, books, movies, and other items that are taking up space, then delete items you no longer need.

• macOS Ventura or later: Learn how to optimize storage space
• Earlier versions of macOS: Learn how to optimize storage space

Move or delete files

If you have another storage device, such as an external drive connected to your Mac, you can move files to that device. Media files such as photos and videos can use a lot of storage space, so you can take steps such as these:

• Move your Photos library
• Move your iMovie library

You can also delete files that you no longer need. For example:

• Delete music, movies, podcasts, or other media, especially if it's media that you can stream or download again as needed.
• Delete files in your Downloads folder. Open this folder from the Dock, or by choosing Go > Downloads from the menu bar in the Finder.
• If you use the Mail app for your email, choose Mailbox > Erase Junk Mail from the menu bar in Mail. Also choose Mailbox > Erase Deleted Items.
• If you use your Mac to back up your iPhone or iPad, you can delete old backups.
• Uninstall apps that you no longer use.
• If your Mac is set up for multiple users, delete users that no longer need access to your Mac. Or ask them to follow the steps in this article after logging in to their user account. Also check the Users folder on Macintosh HD for any undeleted data belonging to deleted users. 
• Check the Users folder on Macintosh HD for any shared files that you no longer need to share.

If you don't want to delete certain files, you might be able to save space by compressing them. Then expand them when you need to use them again."

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

Let us know if you have any questions.

Thank you for using Apple Support Communities.

Take care!