"System Data" is Huge

The System Data is what was previously calle the Other category and is a potpourri of files which can 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: browser, Mail

• Mail messages & attachments

• Fonts, plugins, extensions

• Safari reading list

• iTunes backups

• Crud resulting from jailbreaking your iDevice

• Game data

• Saved data files

• Call history

• Notes

• Media

• Voice memos

• 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

• Files created and modified by other user accounts on your Mac.

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  or OmniDiscSweeper, 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.  

Note: you can empty the Downloads folder after the apps and/or updates that were downloaded have been installed or applied.  Many users have found a couple of Gigabytes of files in their Downloads folder which are no longer needed. 

I don't know what to tell you. Give GrandPerspective a try and see what the largest files are on your drive. It's so easy to use as it's all visual.

Control (right) - click on the item and you can go to that file.

If you've not able to clean out much you might consider an external SSD to store large but seldom used files.

You could get a unit like this 250 GB mini from OWC (MacSales.com):

I use this setup:

Not as small as the mini or esthetically appealing as the mini but can get 1 or 2 TB SSDs that are easy to carry.

Just some food for thought.

Kurt Griffith wrote:

It feels kind of inappropriate that relatively "untouchable" System Data

It's just that Apple tries to over-simply things, then over-complicate things, and finally settles on simply over-confusing things.

The "System Data" actually isn't system data. This is just an "other" category. I don't know why they didn't call it that, or why they didn't call it what it really is, which is local snapshots and "other" data.

It should go away in a day or so, assuming you have not installed any 3rd party software, 3rd party system modifications, 3rd party backups, 3rd party system hacks, etc. If you have done any of that, then who knows?

If you want to explore the over-complicated point of view, you can use Disk Utility, with about 3 layers of non-default settings, to see just how much space each local snapshot it taking, and selectively delete them to free up space immediately.

But normally, this is never a problem and nobody ever notices it unless they absolutely run out of space on the hard drive. And then, someone has to try to explain all of this yet again, or else post something else like a wall full of incomprehensible text and links. That's why I recommend keeping 100 GB free. Since you have a 250 GB hard drive, that is probably going to be difficult. So try to keep 100 GB free on your next computer.

Keep the Apple SSD because if there's another system update or upgrade with a firmware update it won't install on a non-Apple SSD. So you 'll have to install the original SSD, run the update and then reinstall the 3rd party SSD and update again.

It's all normal. But you are out of storage space. You will need to delete some files to make about 100 GB free. It will take a day or two for the storage to be completely freed up.