What to do with the system data problem?

You can get to the bottom of what's in your "System Data" by running a utility like Daisy Disk. It will show you exactly (run it as Administrator). I recently used it and found old iPhone backups, Garmin maps that were leftover from installs to Garmin devices, iOS install files for an iPhone, and some large DMG files. Removed those and saved 50+ GB. Previous instances included old Photos libraries (misclassified as they should have been in the Photos category), folders associated with email software (misclassified as they should have been in the Mail category), and some various user and files associated with various applications (some misclassified).

If you only have 250 GB you will face this problem repeatedly until you replace the computer with one that has decent storage. The only users I know of who can use a 250 GB computer have placed nearly everything they have in iCloud. But that's not for everybody (certainly not for me).

Pål B wrote:

Hmm. According to DaisyDisk the preboot volume is 24.5 GB. Disk Utility says it's 7.15 GB.

You should not touch the Preboot area, it is to ensure your Mac boots properly. If you remove certain items from your Mac, such as Rosetta, that might (or might not) reduce slightly the size of Preboot. The difference in size shown has to do with different bookkeeping in Disk Utility versus Daisy Disk, but none of that 24.5 GB is removable by the user or owner of the Mac unless you want a Mac that won't boot anymore. Some of that 24.5 GB (versus 7.15 GB) is called "System Data" by the Mac while Daisy Disk sorts it slightly differently. But none of that area is user modifiable.

In Daisy Disk, focus your attention on the Users folder, and maybe Applications if there are large things in there you don't use. Also inside Hidden Space you will find snapshots, which in principle can be deleted (I prefer to delete them using Disk Utility, which can also show them) but those usually don't take up much space and they do get recreated by the Mac as it operates.

If Daisy Disk shows "System" as taking up 50 or 60 GB, that is normal. Can't really shrink that. You can only shrink space taken by Applications if you installed non-Apple software. Nothing else should be taking much more than 10 GB (aside from user folders which can be much larger). Your basic problem is a 250 GB disk which is really too small for most usage unless all you do is web browsing and you store most everything in the cloud without mirror copies physically on your physical local drive.

You can move all user Photos and Music libraries to external drives to save space. For me that would be > 100 GB but I have plenty of internal storage so I keep them on the internal drive. And all your documents can be put in iCloud if there is a lot of room going for those. I prefer to keep physical copies on my Mac. I think getting a computer with 1 TB or more is a must nowadays. With 250 GB your options are somewhat limited.

Pål B wrote:

Many people ask and discuss about system data, it is a problem, but it is difficult to get any clear answers.

that’s because there aren’t any.

Apple deceives its buyers when they claim that the hard drive is for example 256 GB - the size of Mac OS. In reality the system takes up much more space than about 29 GB.

The only deception is the definition you have assigned to system data. All of the storage displays with the pretty colors are generated by Spotlight. The different categories (not folders) are generated from a search using Spotlight metadata. Anything that spotlight cannot identify as one of the names categories falls into the catch-all called “System Data”

Why is it so big?

it can be many things. It could just be that Spotlight has misidentified or failed to identify certain data that’s actually your files.

sometimes, it can be local Time Machine snapshots made because you have Time Machine turned on and you haven’t connected a backup drive in several weeks.

It could be system modifications you have installed that are interfering with Spotlight.

It could be overgrown or runaway log files. Those could be from the OS itself, or from third-party software.

what it isn’t is macOS. That’s around 19 GB as you noted.

What can be done to reduce it?

if it is a spotlight issue, you may be able just to re-index Spotlight, but if it is some third-party system modification that’s messing up the spotlight index, that won’t help. It also will not reduce the amount of data used, it just might categorize it correctly as a document or an application or one of the other categories.

I can’t give you a definitive answer on how to reduce it as I don’t suffer from the problem. I don’t think I’ve ever seen my system data get much above 50 GB and I have four times that much free space on my drive.

System Data Problem ?

The most popular way to address this issue is to Start Over from Scratch 

For Apple Silicon computer >> Use Disk Utility to erase a Mac with Apple silicon.

For Apple Intel computers >>   Use Disk Utility to erase an Intel-based Mac followed by How to reinstall macOS

Always make a Time Machine Backup  before  proceeding 

It is also suggest to  Only Migrating the User Account and nothing more.

From what I have seen on these Forums 

xx.xx GB in System Data  seems to be about normal for some but not all 

Generally when this issue is discovered by the User ( you in this case )

It is because the Internal Drive Capacity of this computer maybe on the small size

Meaning, the computer has a 256 GB Capacity Drive or possible the 512 GB Drive Capacity 

Unfortunately, the users' needs for Storage may have grown since the time it was originally purchased 

The implications being that on your next purchase

Spend the Extra Money Up Front on a larger Drive Capacity Drive and add more RAM

This may just Future Proof the computer for your growing needs 

Just saying,  we get what we have paid for 

OP wrote " Apple deceives its buyers when they claim that the hard drive is for example 256 GB - the size of Mac OS. In reality the system takes up much more space than about 29 GB. "

Parsing the wording above

The Operating System in and of itself on this machine uses 21.51 GB - Sequoia 15.5

It is when one chooses to enable Apple Intelligence does an addition 10.7 GB will get added

So I tend to disagree with the notion of a deception but rather a misunderstand by many

Yes, User Data, technically has a mount point of /System/Volume/Data

At that point it becomes part of the Operating System

Further, /System/Volume/Data is a mesh mash and a collation of may things including the User Account ( Home folder )

Agreed, there is no Definite answer to exactly what is included making to whole issue unanswerable or not acceptable to some

To see a far more accurate view of Space Used versus Free

I rely on Disk Utilities as most other reporting methods are not nearly as accurate

As suggested I have erased 3 different Macs and reinstalled all to reduce the system data storage amount. Only cure I have found. However, over time it may grow once again.

Pål B wrote:

I have come back to Mac but if I remember correctly it did not exist before, many years ago. Not like that. Why is it so big? What can be done to reduce it?

Many years ago, operating systems were smaller and did less. The operating system for an Apple IIe fit on a 5.25" floppy that had a maximum capacity of, if I recall correctly, 1.2 megabytes. Mac software started on 3.5" disks and then moved to CDs. As the OS becomes more complex, it gets bigger. As BDAqua says, it's only going to get bigger.

Fortunately, external hard drives have gotten progressively bigger and cheaper. The first one I bought was 70 megs and cost me almost $500, on sale.

It isn't easy to find but Disk Utility does show a Preboot size, but it counts things up different than Daisy Disk so Daisy Disk shows that "Preboot" as about 25 GB. I see this on multiple Macs and the OP also sees it. Either way, that is not user shrinkable space, regardless of how it is counted or binned. I think Disk Utility counts it at 7 GB and the other 18 GB is bokkkept as "System Data," another reason "System Data" sometimes is larger than people expect. I think these details can vary with Mac model and with specific version of the MacOS.

Actually I find the Operating Systems themselves have not grown that much

Since the advent of Apple Intelligence integration into the OS

Yes add an additional 10 + GB above and beyond the OS

I think that everyone understands that there is limit to the storage capacity of your HD: you get what you pay for. But the issue of System Data seems different, for me. For months I had the "system data endless calculating" issue, for which I also had direct assistance from Apple (to no avail), and in the end the problem suddenly disappeared without me understanding why. Moreover, I saw numbers associated with "system data" go up and down (a few GB) in a few days without me doing anything significant with my Mac: no adding or deleting large files etc. I think that those "hard to understand" variations in System Data, and sometimes their exaggerated growth are what makes people irritated. It is quite clear to me and everyone else that if I have a 256GB HD I have a significant limit but this is not the real issue, in my opinion.

Maurizio

My friend, for your perusal…

System 7 had a larger memory footprint than System 6. System 6 could boot the system from a single 800k floppy disk and uses about 600 KB of RAM, whereas System 7 used well over one megabyte. It was some time before the average Mac shipped with enough RAM built-in for System 7 to be truly comfortable. System 7 was the first system release that could no longer be usefully run on floppy-only systems.

🙈🙉

There has been sufficient advice offered, by several different contributors, for the User ( you ) to make an informed and educated choice what remedial actions are  required for this computer.

The suggestions have been put forth on a volunteer basis, in good faith and in the best interests of the computer.

While some individuals visit these forums seeking technical solutions to specific Apple-related issues, there are also those who seek recognition or fame through their contributions. 

Pål B wrote:

You don't have this issue on Linux though it have similar features. Caches, log files, temp files, config files, and all that is fine. I'm wondering why it can become so big on Mac OS. Is it system snapshots that can be removed ( I don't use Time Machine)? Siri, that I don't use?

macOS is not Linux. Comparing apples to ball bearings is only moderately useful.