What Is "Other" Storage on a Mac?

Open Apple menu → System Settings → General → Storage and you get a coloured bar with sensible categories: Applications, Documents, Photos, Music. Then a grey block labelled System Data — Other in older versions — holding 30, 60 or occasionally 150GB, with no way to open it.

macOS is not hiding a leak. It is refusing to classify files that do not fit its categories, and on a Mac most of those files belong to one of four things.

What is actually in there

The first row is the one that surprises people, and it is worth understanding before you go looking for something to delete.

Local snapshots: large, alarming, and mostly self-managing

If Time Machine is enabled, macOS takes an hourly snapshot of your disk and keeps it locally — so you can restore a file even when the backup drive is at home. These snapshots are stored on the same volume and counted as System Data.

They can reach 80GB. They are also purgeable: macOS deletes the oldest ones automatically as soon as free space runs short, and always before you actually run out.

That is why Finder and Storage Settings disagree about your free space. Finder counts purgeable data as used; Storage Settings counts it as available. Storage Settings is telling you the more useful truth.

To see what you have, open Terminal and run:

tmutil listlocalsnapshots /

To reclaim it immediately rather than waiting for macOS to do it:

sudo tmutil thinlocalsnapshots / 10000000000 4

That asks macOS to free roughly 10GB with urgency level 4. Your Time Machine backups on the external drive are untouched — only the local copies go.

Most of the time, doing nothing is the correct choice here.

Use the built-in Storage Management screen

Before going anywhere near Library folders, use the tool Apple ships.

In System Settings → General → Storage, the list below the bar chart expands each category. The useful entries:

• Applications — sortable by size, with last-used dates.
• Documents — includes a Large Files view and a Downloads view. The Downloads folder is routinely 10–30GB of installers nobody needs again.
• iOS Files — old iPhone and iPad backups. Each is 5–60GB, and people frequently have three generations of a phone they no longer own.
• Trash — with the option to empty items automatically after 30 days.

The recommendations at the top — Store in iCloud, Optimise Storage, Empty Trash Automatically — are safe and do what they say.

Clearing old iOS backups is usually the biggest single win on this screen.

Clear caches, carefully

Caches are the second-largest contributor. They are safe to clear because every file in them is regenerable by definition.

Quit your applications first, then in Finder press Shift+Cmd+G and go to:

~/Library/Caches

Delete the contents of the folder, not the folder itself. Apps rebuild what they need on next launch. Expect 5–20GB on a Mac that has been in use for a few years, with browsers and development tools the largest entries.

Do not touch anything else in Library. Specifically:

• ~/Library/Application Support — app data, licences, local databases
• ~/Library/Containers — sandboxed app data, including Mail
• ~/Library/Preferences — every app setting you have ever configured

Deleting these loses real data and is the most common way a cleanup turns into a reinstall. There is also a system-level /Library/Caches — leave that one alone; the gain is small and the risk is not.

The specific offenders worth checking

Certain applications accumulate far more than their function suggests:

• Xcode — derived data and simulators routinely exceed 50GB. Clear from Xcode → Settings → Components, and delete ~/Library/Developer/Xcode/DerivedData.
• Docker — images and volumes are invisible to Finder. Run docker system prune -a if you know what it will remove.
• Photos — check for a second library file; .photoslibrary bundles do not always appear where you expect.
• Mail — downloads attachments permanently. Mail → Settings → Accounts → Download Attachments → None stops it.
• Browsers — Chrome in particular keeps a substantial disk cache, separate from the memory behaviour covered in Chrome using too much RAM.

What not to do

• Do not delete files in /System or /private. System Integrity Protection blocks most of it, and what it does not block, you need.
• Do not run cleaner apps with “deep clean” modes. The built-in screen finds the same large files. Aggressive cleaners have repeatedly removed language packs and support files that applications depend on.
• Do not delete ~/Library wholesale. It is your entire user configuration.
• Do not panic about purgeable space. If Storage Settings says you have room, you have room — macOS will free the snapshots when it needs to.

Realistic expectations

Removing old iOS backups, emptying Downloads and clearing user caches typically recovers 20–60GB on a Mac that has never had it done, and takes twenty minutes. Local snapshots account for most of what remains, and leaving those alone is correct.

If System Data is still enormous afterwards, run the Large Files view in Storage Management and sort by size. On a Mac that has held out this long, the answer is almost always one specific thing — a virtual machine image, a video project, or a developer toolchain — rather than a broad accumulation.