Mac Storage "Other" Explained (It's Called System Data Now)

You open storage settings, see a fat grey bar labeled System Data, and there is no More Info button next to it. Every other category tells you what is inside. This one just sits there taking up 60 GB.

First, a naming correction that most articles on this topic have not made: the category is called System Data. Apple renamed it from “Other” in macOS Monterey, back in October 2021. If a guide you are reading still calls it “Other” without noting the rename, it has not been updated in five years, which tells you something about the rest of its advice.

Everything below was verified against macOS Tahoe 26.5.

Where to find it

Apple menu → System Settings → General → Storage.

On macOS Ventura 13 and later that is the path. Before that it lived under About This Mac → Storage.

You will see Applications, Documents, Messages, Music Creation, Podcasts, Trash, TV, and System Data. Most have a More Info button. System Data does not, and that omission is the whole reason this article exists.

What is actually in there

Apple’s own definition is deliberately broad: files that do not fall into any of the other categories, primarily “files and data used by the system, such as log files, caches, VM files.”

In practice, on a machine that has been in use for a year, System Data is mostly these five things:

APFS local snapshots. Time Machine saves a snapshot of your startup disk roughly every hour and keeps it for 24 hours. macOS also takes one before installing any system update. On this Mac right now there are three, all from OS updates.

iPhone and iPad backups. These live in ~/Library/Application Support/MobileSync/Backup/ and they are large. An old backup from a phone you no longer own can sit there for years.

Caches and logs. Application caches, system logs, temporary database files, interrupted downloads, staged macOS updates, Safari website data.

Mail attachments. Mail downloads attachments according to your account settings and keeps them locally.

Virtual memory and swap. Grows under memory pressure, shrinks afterward.

Why three tools give you three different numbers

This is the part that makes people think something is broken. It is not. There are three separate accounting systems at play, and they genuinely disagree.

Here is the same disk on this machine, measured three ways within the same minute:

df -h /                       460Gi size    16Gi used   210Gi avail
df -h /System/Volumes/Data    460Gi size   214Gi used   210Gi avail
diskutil                      Container total 494.4 GB
                              Volume used     230.1 GB
                              Container free  225.8 GB

Three numbers for “used”: 16 GB, 214 GB, 230 GB. All three are correct. They measure different things.

Reason one: the system volume is sealed. Since macOS Big Sur, / is a read-only signed snapshot. It reports about 16 GB and it will report 16 GB no matter how much you delete. Your files live on /System/Volumes/Data. Any guide that tells you to run df -h / is showing you a number that never changes.

Reason two: APFS shares free space across volumes. Both volumes above claim the same 210 GB available, because they are drawing from one container. You cannot add per-volume numbers together, which is a mistake that inflates totals badly.

Reason three: purgeable space. Apple defines purgeable as space macOS can free when needed by removing files automatically. You cannot delete purgeable files yourself; the system removes them as space is required. Finder’s “available” figure includes purgeable space. df does not. That single difference explains most of the gap people notice between what Finder promises and what a copy operation actually succeeds at.

For developers, the API that matches Finder’s optimistic number is volumeAvailableCapacityForImportantUsage, not volumeAvailableCapacity.

The snapshot trap

This one deserves its own section because it is where most “System Data is eating my disk” panic comes from, and where cleaner apps make their easiest sale.

Apple states it directly: your Mac counts the space used by snapshots as available storage. Snapshots are removed automatically as they age or when the space is needed for something else.

Read that again, because it inverts the usual conclusion. The 40 GB of snapshots your cleaner app just offered to reclaim is not 40 GB you are missing. macOS already counts it as yours and will hand it back the moment you need it. Deleting them manually mostly means giving up your ability to roll back a bad update, in exchange for a number in a settings pane looking nicer.

If you genuinely need the space right now, the supported tools exist:

# See what snapshots you have
tmutil listlocalsnapshots /

# See just the dates
tmutil listlocalsnapshotdates /

# Ask macOS to reclaim a specific amount by thinning, urgency 1-4
tmutil thinlocalsnapshots / 10000000000 4

thinlocalsnapshots is the right tool over deleting individually, because you are asking macOS to free a target number of bytes and letting it decide which snapshots to give up.

Note that local snapshots only exist if you are on APFS with Time Machine’s automatic backups enabled. No Time Machine, no hourly snapshots, and this whole section does not apply to you.

What you can actually delete

Working from safest and largest:

Old iOS backups. Connect the device, open the Apple Devices app (or Finder), go to General, click Manage Backups, right-click any backup and delete. Backups from devices you no longer own are pure waste. This is often the single biggest win.

Mail attachments. In Mail, select messages and use Message → Remove Attachments. One warning that matters: for IMAP accounts this deletes the attachment from the server too, and it is not reversible. Do not do this to anything you might need again.

Downloads and Trash. Obvious, routinely forgotten. Turn on the setting that empties Trash automatically after 30 days.

Store in iCloud and Optimize Storage. Both in the Storage pane. They move Desktop and Documents to iCloud and keep only recently opened files locally, and remove watched Apple TV content and old email attachments when space runs short.

Safe mode boot. Apple documents that booting into safe mode clears certain system caches, which are then rebuilt as needed. This is the closest thing to an official “clear System Data” button, and it is worth trying before anything drastic.

What not to do

Do not delete things inside /System/Volumes/Data/private/var/ by hand. You will find large files there. They are in use.

Do not buy a cleaner app to reclaim snapshot space. See above. You already have that space.

Be sceptical of any tool that shows a dramatically bigger number than the Storage pane. Given the three accounting systems described earlier, the easiest way to produce an impressive “reclaimable” figure is to count purgeable space, snapshot space, and shared APFS blocks as if all three were separately recoverable. None of them are. I am describing what macOS actually counts, not making a claim about how any specific app computes its figure, since none of them publish that.

There is one popular fix I deliberately left out: the advice to rebuild the Spotlight index with mdutil to correct a wrong System Data figure. I could only find it on cleaner-vendor blogs and forum posts, with nothing from Apple supporting the idea that the Storage pane derives its numbers from the Spotlight index. It might work. I am not going to tell you it does when I cannot show you why.

Watching the number instead of panicking at it

System Data grows and shrinks on its own. That is the design. It swells after an OS update (snapshot), after a big Xcode build (caches), after you plug in a phone (backup), and then macOS quietly gives most of it back.

The reason it feels alarming is that you only ever look at the Storage pane when something has already gone wrong, so every reading you have ever taken of it is from a bad moment. You have no idea what normal looks like on your own machine.

That is the actual argument for keeping storage visible somewhere you glance at daily. Storage Peek puts free space in the menu bar and can alert you at a threshold you pick, so a slow leak is something you notice at 40 GB free rather than at 2 GB during a deadline. It is $2.99, one time, no subscription.

To be straight about what it will not do: it reads the filesystem, so like every treemap analyzer it cannot see purgeable space or tell you what deleting a snapshot would truly free. Nothing can, short of deleting and re-measuring. What it gives you is the trend line, which is the part you are actually missing.

FAQ

Is System Data the same as Other?

Yes. Apple renamed the category in macOS Monterey (October 2021). Older articles and older machines call it Other, and Apple’s current documentation still writes “System Data (or Other)” for readers on old systems.

Why is my System Data 100 GB?

Most likely an old iOS backup, accumulated snapshots after a recent update, or Xcode. Check ~/Library/Application Support/MobileSync/Backup/ first, it is the biggest single item on most machines that have it.

Can I delete System Data directly?

No, and there is no supported way to. It is a label covering many different managed things, not a folder. You reduce it by deleting the specific things inside it.

Why does Finder say I have more free space than df?

Finder includes purgeable space in its available figure. df reports only genuinely free blocks. Neither is lying, they answer different questions.

Will restarting reduce System Data?

Sometimes, modestly. A restart clears swap and some temporary files. Safe mode does more, because macOS clears certain system caches during that boot.

Should I worry if it keeps growing?

Only if free space is actually running low. The category growing while you still have 100 GB free is macOS using space it will release when asked.

The short version

It is called System Data, not Other, and has been since 2021. It is mostly snapshots, caches, and iPhone backups. Your Mac already counts snapshot space as available, so most of the scary number is not missing. Delete old iOS backups first, use safe mode to clear caches, and measure free space on /System/Volumes/Data rather than /, because the system volume is sealed and its number never moves.