My Mac's System Data is using 210 GB out of 250 GB

One of 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 

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 may have 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 

Follow ON if you please

This is the Long Version

System Data technically, has a mount point /System/Volume/Data 

With a mount point of /System , this Volume is part of the Operating System and can not be deleted - full stop

It is where your User Account (s)  was made when the computer was First Setup as New

We can point-out what may be included in this Volume

Though, how to reduce this is a long and complicated trial and error process

The category is a collection of files which include:

• System temporary files

• macOS system folders

• Archives and disk images (.zip, .iso, etc. - often found in the Downloads folder)

• Personal user data

• Files from the user’s library (Application Support, iCloud files, screensavers, etc.)

• Cache files

• Fonts, plugins, extensions

• Other files that are not recognized by a Spotlight search

• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package

•Time Machine Backup Snapshots that have not been transferred to the TM Backup Drive 

• Locate backups of your iPhone, iPad, and iPod touch.

•Was there or is there another user account currently or previous used on this computer ?

Has the account been removed ?

Did the removed user account ’s Home Folder also been removed ?

Delete a user or group on Mac

• If using a Cloud Based Services to sync files,  like iCloud and One Drive. These can also occupy space on the Internal Drive 

They can be located anywhere on your hard drive. The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use either of these two free apps, 

GrandPerspective , to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.  

Another Application to find and analyze what is using space on the drive is Etrecheck >> Clean up

Maybe: About Time Machine local snapshots - Apple Support

You don't say which computer model.

Also worth reading: Free up storage space on Mac - Apple Support

That's a bit of a nuclear option. If the computer is getting full now, you erase it, and then reload your things again, it will likely be full again.

Snapshots are structured to use available space, so I question the starting premise that the drive is actually full because of user items. It could be cozy because of snapshots and the computer is behaving as expected.

If the drive is full because of user items then the solution is to archive some items on external storage (+ a backup of that storage) so there's room on the internal drive again. I have all my movies on external storage because, frankly, I don't watch a movie more than once every 5 years or so.

Agree to a certain extent

OP seems to have mentioned 210 GB of 251 GB, implied Total Drive Capacity

Kind of plays into the last part in my original suggestion

Not to labour on the point but a drive of 256 GB is akin to the days apple sold the entry level 128 GB drives

Yes 256 drive are Manageable provided it is frequent monitored by the user. 😎

One problem is Apple's drives are so expensive. Even putting in a 500GB drive adds another couple of hundred dollars. For a couple of hundred dollars you can buy two 4 GB HDDs. I think many people are having to fundamentally rethink how they use their storage. It used to be you kept everything you had on one internal drive and you had an external backup (or lived dangerously and did not have the backup). 10+ years ago you could easily and inexpensively upgrade a drive on any computer so even if you needed 1TB you could have one without adding a lot to the cost of a computer.

Now the strategy is to keep what you need for daily use on a computer and put what you don't on external drives or cloud storage. I am about to downsize from a 1TB internal drive to a 500GB internal drive. I'll just keep my media on an external drive plugged into my new Mini. I already store photos on archive drives, and I'll move off some other files I rarely look at. Of course with a Mini I am using a desktop computer so having a storage drive doesn't really add much vs. using notebook. Still, if your main storage user on a notebook is photos, who looks at photos all that frequently? I have mine on external drives and when I want to look at Christmas 2002 I just attach my 4TB photos drive. I keep about 10 GB of most recent photos on my computer.

Incidentally, my wife bought a 256 GB MacBook Pro back in 2013. When we went shopping for a Mac a few months ago we looked at her drive usage and it was about half full. So she really doesn't need anything larger (honestly, if it comes down to it should could transfer her 40 GB of photos to one of my drives).

I have to agree with Owl on this one, sometimes a fresh erase and reinstall is the answer when it comes to system data. But what would be suggested is not to bring back the full Time Machine backup back.

If you backup most of your data on iCloud like iCloud Drive, Contacts, Pictures, etc (you can check by clicking the apple logo>system setting>Your name top left>iCloud>See all>Make sure everything you want is green)then it will come back with you apple account

Keep in mind you will have to reload your apps manually but that's an easy task by opening the App Store>Click on you name at the bottom left, then purchase by You will be all your purchase and free app>Click on the cloud besides it and they will reload on your device

Bringing back the backup, like others mention may bring back the issue. It all depends if you're ok with just everything that's on iCloud and reloading the app.

If you are, follow Owl article to erase and reinstall macOS keep in mind erasing takes 2 minutes but reinstall macOS takes around 45min to an hour and half.

A fresh erase resolves a lot of storage issue, if your ok with that option!

Also keep in mind that you can also bring back data manually from your Time Machine backup versus the whole backup if you see things are missing.

@ Limnos

Thank you for engaging with me and sharing your obviously long experience with Apple Computer

Yes, at time of purchase we are presented with choices in regards of computer configurations ie Drive Capacities and RAM to be specific

Other factors come into play like costs and the uses' budget, not to mention the Want to be seen with an Apple Computer for social standing purposes

Unfortunately the general rule of many people when making a decision on the purchase is looking at the needs for the " Moment " . Tomorrow or next year or year after does not enter the picture at all

Needs grow, circumstances change and workflow / demands change

Now we are stuck with our choices we made 2 years ago and without having the foresight to project what their needs may become

At the end of the day and to use the Metaphor

If you what to have a high performance vehicle, you to need to have the money to buy high octane fuel

Yes, one can run the vehicle on a lower grade fuel but it will never perform as expected

Just saw you reply now.

Not through Apple Notifications

Am off line until tomorrow

Willl back to you then.

Limnos wrote:

For a couple of hundred dollars you can buy two 4 GB HDDs.

Meant to say 4TB!

I am running a system much older than Sonoma but I find that sometimes simply restarting the computer can clear up a few GB of space.