Disclosure up front: Purplelink, which publishes this site, makes one app mentioned here, Legroom, a menu-bar app sold as a 7-day trial and a $9.99 one-time purchase. It is young, it does not clear the biggest suspects below, and I say where other tools do better. I ran the commands and looked at the folders named here on my own Mac on October 12, 2026 (macOS 27.2). Everything else is read from Apple's pages and the vendors' own pages on the same day, and where a page did not say something I write "not stated" rather than guess.
The short answer
Apple's own wording is that System Data is a general category that measures the storage space used by all Apple and third-party files that do not belong to a more specific category (Apple's page on freeing up storage, published July 6, 2026). It is whatever macOS does not file under Applications, Documents, Photos and the other labelled bars, so it can hold app data, caches, Time Machine local snapshots, device backups and system files at once.
To find your cause, open System Settings, General, Storage, then look inside your Library folder for the biggest folders, as in the next section. The usual suspects are app data and caches, Time Machine local snapshots, old iPhone or iPad backups, the models behind Apple Intelligence, and a media analysis cache that some people find at 100 GB or more. None of the tools in this guide shows System Data as one tidy item either, so expect to work through folders.
Find where it went
- Look at Apple's bar. In macOS 13 or later, choose Apple menu, System Settings, General, Storage (Apple's Optimize Storage page). The number for System Data there is a computed leftover, so it can differ from what you add up folder by folder in Finder.
- Open your Library folder. In Finder, hold the Option key and choose Library from the Go menu (Apple's page on going to a folder). Most of the user-level space is under it.
- Make Finder show folder sizes. In a window set to List view, choose View, Show View Options (Command-J) and tick Calculate All Sizes (MacRumors describes the steps). Finder remembers the setting per folder, and calculating every size can be slow.
- Or ask Terminal. This lists the ten biggest folders directly under your Library, and it took about 40 seconds on my Mac:
du -sh ~/Library/* 2>/dev/null | sort -h | tail -10. It only reads; it deletes nothing.
Not everything is under your own Library. Time Machine snapshots and the sleep image live elsewhere, as the next section explains.
The usual causes, and whether each is safe to remove
| Cause | Where it lives | Safe to remove? |
|---|---|---|
| App data and containers | ~/Library/Application Support, ~/Library/Containers, ~/Library/Group Containers | Only for apps you have already uninstalled. For an app you still use, this is its data: settings, databases, downloads. |
| App caches | ~/Library/Caches | Usually rebuilt by the app. Quit the app first. Apple says macOS also clears caches and logs that are safe to delete when it needs space. |
| Time Machine local snapshots | On the startup volume, listed with tmutil | Let macOS manage them in almost every case. See below. |
| iPhone and iPad backups | ~/Library/Application Support/MobileSync/Backup | Yes, if you no longer need the backup, and delete through Finder, not by hand. See below. |
| Apple Intelligence models | Managed by macOS | Not by hand. Apple's page does not describe removing them. See below. |
| Media analysis cache | ~/Library/Containers/com.apple.mediaanalysisd/Data/Library/Caches | Apple does not say. Reports exist of people clearing it. See below. |
| Swap and the sleep image | /private/var/vm | No. macOS manages these. Leave them alone. |
Time Machine local snapshots
This is the usual reason deleting files does not free space. Apple says Time Machine also stores some of its backups on your Mac, and that your Mac counts the space used by snapshots as available storage. Time Machine deletes them as they age or as space is needed. To see them, run tmutil listlocalsnapshots / in Terminal; on my Mac it listed none. Apple's documented way to remove them by hand is to turn off automatic backups in Time Machine settings and wait a few minutes. The tmutil man page also lists deletelocalsnapshots and thinlocalsnapshots, and I did not run either.
iPhone and iPad backups
Finder stores device backups at ~/Library/Application Support/MobileSync/Backup, and Apple's page on locating and managing backups says to manage them from the device in Finder: General, then Manage Backups, where you can delete, archive or reveal a backup. That folder was empty on my Mac, since I do not back up devices to it. Do not delete the folder's contents by hand unless you are sure you do not need the backup, because a deleted backup cannot be restored.
Apple Intelligence models
Apple's page on Apple Intelligence requirements, published September 14, 2026, lists up to 14 GB of storage on a Mac with an M3 or later and at least 12 GB of unified memory, and up to 8 GB for other supported devices. 9to5Mac reported on October 5, 2026 that Apple Intelligence needed up to 7 GB on macOS 26 and that Apple says up to 14 GB on macOS 27, and that an open-source tool called RemoveMacAI claims to reclaim 12 GB or more. I have not run that tool, and the Apple page I read does not describe a way to remove the models while keeping the feature, so I do not recommend removing them by hand.
The media analysis cache
Photos and related features use a background process called mediaanalysisd, which keeps a cache in a container folder. Michael Tsai's blog collects reports of it at 15 GB or more, with some people reporting 50, 80 or 140 GB, and says the cause is unknown. In an Apple Community thread from May 2025, one person deleted a 21 GB subfolder and reported freeing about 30 GB, and other replies advised deleting it; nobody there reported a problem. A popular r/MacOS thread this week reported 137 GB in the same place (thread, read October 12, 2026).
Apple has published nothing about this folder, and I could not confirm from those pages whether macOS rebuilds what you delete. So this is a report, not advice. If you try it, quit the apps that use Photos, remove only what is inside the cache folder and leave the container folder in place. On my Mac the folder is 491 MB and holds different files from the ones those reports describe, so a big number is not universal.
What to leave alone
Do not touch /System, and do not delete swap or the sleep image in /private/var/vm. The sleep image on my Mac is 2 GB, and macOS manages it. If you cannot name what a folder is, find out before you remove it.
What I measured on my own Mac
For scale, this is a Mac running macOS 27.2 with a 228 GB startup disk and 52 GB free. Under my Library: Application Support 11 GB, Group Containers 7.4 GB, Containers 4.8 GB, Caches 3.5 GB, the media analysis cache 491 MB, and device backups none. It is an ordinary working Mac, and none of its folders is anywhere near the 100 GB reports, which is the point: System Data is large on some Macs because of one specific thing, and the folders above are how you find out which.
Tools that show or clear it
A folder map is the fastest way to find the big folder, and the guide on disk space analyzers and cleaners for Mac compares them in detail. In short, from each vendor's own page read October 12, 2026:
| Tool | Price | What its page says |
|---|---|---|
| DaisyDisk | $9.99 once | A visual map; requires macOS 10.13 or newer; a free try is offered |
| GrandPerspective | Free from SourceForge, or $2.99 on the Mac App Store | Version 3.8.1 requires macOS 14 or higher |
| OmniDiskSweeper | Not stated on its download page | Finds large files and moves them to the Trash |
| OnyX | Free, donations optional | The current version is for macOS 27 and Apple silicon only; older Macs need an older version |
| Mole | Free command-line tool (GPL-3.0); a separate Mac app is proprietary | Cleans, uninstalls and analyzes disk use; requires macOS 12 or newer |
| StorageRadar | $9.99 once (Core), $19.99 once (Developer) | Requires macOS 15 or newer; its Dry Run shows exact paths and sizes before anything is applied; sold on the Mac App Store and as a direct build |
| Legroom (mine) | Free 7-day trial, then $9.99 once | macOS 14 or later; shows what grew over 1, 7 or 30 days; previews each rule before it removes anything |
If someone says Mole or OnyX felt too technical, as the r/MacOS poster did, a visual map such as DaisyDisk or GrandPerspective is the gentler route. I did not test any of these for this guide; the table repeats what each page states.
Where Legroom fits, and where it does not
Legroom lives in the menu bar and shows free space on the startup disk. Its daily "what grew" comparison over 1, 7 or 30 days names the folders that filled the disk, which helps when something quietly grew over a week. Cleaning is by rules you make, and each rule previews what it would remove before you approve it. Its Space Map draws folders as rings.
The limits matter for this problem. Legroom does not offer Apple's media analysis cache for cleaning, it does not remove Apple Intelligence models, and because it does not scan as an administrator it reports space such as local Time Machine snapshots as not measured, and shows the Terminal command as text instead of running it. So it will not find or clear the 100 GB folder from the reports above. A folder map such as DaisyDisk will find it, and you then decide what to do. Pick Legroom if what you want is a free-space readout, a view of what grew and cleaning rules you approve first, and you accept a young product from a one-person studio.
Frequently asked
What is System Data on a Mac?
Apple describes System Data as a general category that measures the storage space used by all Apple and third-party files that do not belong to a more specific category (Apple's page on freeing up storage, published July 6, 2026). It is not one folder you can delete. It is whatever macOS does not file under Applications, Documents, Photos or the other labelled categories.
Can I delete System Data?
Not as a whole, because it is not a single item. Some things that land in it are safe to remove, such as old device backups and the app data of apps you have already uninstalled. Others should be left to macOS, such as swap and the sleep image. This guide lists each common cause and whether it is safe to remove.
Why did System Data grow to more than 100 GB?
The causes people report most are a large media analysis cache, many Time Machine local snapshots, old iPhone or iPad backups, orphaned app data and, on newer Macs, the models Apple Intelligence needs. Apple lists up to 14 GB for Apple Intelligence on Macs with an M3 or later. The media analysis cache has been reported at 15 GB to 140 GB, but Apple has published nothing on it and the cause is unknown.
Is it safe to delete the mediaanalysisd cache?
Apple does not document it, so I cannot say it is safe. People in an Apple Community thread report deleting it and freeing tens of gigabytes, and nobody there reported a problem, but that is not Apple's guidance. If you try it, quit the apps that use Photos first and remove only the contents of the cache folder, not the container itself.
Does Legroom clear System Data?
Partly, and not the biggest suspects. Legroom shows which folders grew over the last 1, 7 or 30 days and previews each cleaning rule before anything is removed. It does not offer Apple's media analysis cache for cleaning, it does not remove Apple Intelligence models, and it reports Time Machine snapshot space as not measured.
Related guides
- Best disk space analyzer and cleaner for Mac (2026): the tools above compared in detail.
- Show free disk space in the Mac menu bar: keep the free number in view.
- Legroom, all products and all guides.