Huge System Data

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.

There is Purgeable Space and there is Empty Space.

Purgeable Space which is Controlled by the Operating System. When the Operating Systems decides the computer needs additional Empty Space, it will move a portion of the Purgeable to Empty space

AFAIK - there is no User Actions to hasten this transition from Purgeable to Empty Space

It can day or longer before this will occur  

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.  

Often caused if the Time Machine Drive has not been attached  to the computer and TM Backup is set to run on a Schedule. 

TM Backup will make Snap Shot on the Internal Drive awaiting the TM Backup Drive to be attached. 

Only then will the Snaps Shots be transferred to the External Drive and deleted the Internal Drive.

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.

If you have the same problem that I had a couple of months ago (which sounds likely), then all I can recommend is what worked for me. I started an online chat session with Apple about this, and within 5 minutes I was set up with a senior tech support guy who spent an hour and a half with me on line, holding my hand through a large number of (to me) complicated steps that I do not really understand, with the net result that my System Data dropped from 400 to 41 GB. He said anyone with this problem should reach out to Apple as there is no app, utility, or other ready-made answer, as every case will be individual. We still don't know quite what caused my System Data to get so huge, but at least (for the time being) the problem is solved. By the way, I do not have Apple Care and my 2021 MacBook Pro is of course long out of warranty

Very disappointed in Apple. I bought a 512GB Macbook Air. I spend &10.00 a month for icloud+. 385GB of System data that I can't send to icloud and no way to delete. Had to return it and pay $250.00 extra for a 1TB drive. C'MON MAC!!!!

FYI iCloud was not designed nor intended to like an external drive

Where you off load unneeded large data files in order to make space on the internal drive

icloud is only a conduit to have data in iCloud to you can sync the data to other devices that also use the same Apple ID account

Update - a few months later and System Data was up to over 400GB. Back to Apple online chat, back to senior technical support, ended up with a supervisor of senior technical support, who told me that this is a common problem which has been known for about 10 years but nobody has come up with a way to prevent it. He advised me to completely wipe my computer and reinstall everything, which I did after making backups on an external drive and with Backblaze. At the end my System Data was back down to 40GB, but I have lost 10 years of emails - they seem not to have been copied to the external drive, can't be found on Backblaze, and I can't even retrieve them from my last Time Machine backup from May (I thought have 3 separate backups would be enough, but apparently not). And a week later my System Data is back up to 150GB and tech support will not return my calls. I have been buying nothing but Apple computers since 1986 but am beginning to think it is time to change if I cannot find a way to run a functioning computer (I have an M1 MacBook Pro, with 1TB or storage that gets completely full when the System Data reaches about 400GB or so).

Welcome and thank you for the update

So problem solve

My System Data is 400GB. So if you can't manage the contents of this category, what can you do other than buy a new computer with bigger SSD?

You are welcome to follow the previously suggested steps to OP

Me too