Your Startup Disk Is Almost Full: What to Do

The warning arrives at an inconvenient moment, usually while saving something. It also arrives earlier than people expect — frequently with twenty or thirty gigabytes apparently still free.

That is not a bug. macOS needs working room: swap files for virtual memory, staging space for updates, and room for APFS snapshots. It asks for that room before it runs out, because running out properly makes the Mac very unpleasant to use.

Find out what is actually using the disk

Apple menu → System Settings → General → Storage (Ventura and later; older versions put the equivalent under About This Mac → Storage → Manage).

Wait for the bar to finish calculating, which takes a minute or two on a large disk. Then read it, because the right fix depends entirely on which segment is largest.

The macOS segment is not reclaimable and is not a problem. Fifteen to twenty-five gigabytes is normal.

The quick wins, in order

Do these before anything clever. On a Mac that has never been cleaned they frequently resolve the warning on their own.

1. Empty the Bin. Finder → Bin → Empty. Check whether external drives and iCloud Drive have their own Bin holding more.
2. Clear Downloads. Open the folder, sort by size, and delete the installers. Years of .dmg files is a standard finding, and each one is a full copy of an app you already installed.
3. Delete old iOS device backups. In Storage, find the entry for iOS files or device backups and remove anything belonging to a phone you no longer own. Deleting a backup never affects the device.
4. Use Review Files. The Storage screen’s file browser sorts by size across your whole home folder, which finds the forgotten 40GB video project faster than browsing Finder does.
5. Restart. This clears swap and prompts macOS to thin snapshots. Expect a few gigabytes, not a miracle.

When System Data is the largest segment

This is the common case on a Mac that is several years old, and the usual culprit is local Time Machine snapshots. macOS takes hourly APFS snapshots of the boot volume whenever Time Machine is on, whether or not a backup drive is attached.

List them in Terminal (Applications → Utilities → Terminal):

tmutil listlocalsnapshots /

To remove them all:

tmutil deletelocalsnapshots /

Before running that, confirm you have a real backup on an external drive or network destination. If local snapshots are your only recovery path, deleting them means a file you removed by mistake this morning is gone for good.

System Data holds more than snapshots, and what is inside Mac System Data and how to clear it breaks down caches, logs, Mail attachments and developer folders individually.

The purgeable space confusion

Finder and System Settings regularly disagree about free space, sometimes by tens of gigabytes. Neither is lying.

macOS classifies some data as purgeable: local snapshots, cached iCloud files, and other content it can delete on demand when something needs the room. Storage Settings counts purgeable space as available. Finder counts it as used.

The practical effect is that a Mac which looks full often frees space automatically the moment a large file genuinely needs it. The mechanism and how to force it are covered in how to free up purgeable space on a Mac.

Photos and iCloud Drive

A photo library is frequently the second largest thing on a Mac after the system, and it has a setting most people never change.

In Photos → Settings → iCloud, Optimise Mac Storage keeps full-resolution originals in iCloud and smaller versions locally, downloading the original when you open or export one. It needs enough iCloud storage to hold the library, which is the catch.

The same logic applies to Desktop and Documents through System Settings → Apple Account → iCloud → iCloud Drive. Both move the problem to iCloud rather than solving it, and both are genuinely useful if your iCloud plan has room.

If the real problem is iCloud rather than the Mac, what to delete first when iCloud storage is full covers that side.

Uninstall apps properly

Dragging an app to the Bin removes the app bundle and leaves its support files behind. For most apps that is a few megabytes and not worth caring about.

For a handful it is not. Xcode’s derived data and simulators routinely occupy tens of gigabytes in ~/Library/Developer. Docker keeps a single large disk image regardless of how many containers you run. Virtual machine files are whatever size you allocated. Check for these specifically if you have ever installed them.

What not to do

• Do not delete anything in /System or /Library at the root level. System Integrity Protection blocks most of it, and what you can reach you can break.
• Do not turn Time Machine off to stop snapshots unless you have another backup. A Mac with no backup is a worse problem than a full disk.
• Do not delete /private/var/vm. That is swap. macOS recreates it, typically at a worse moment.
• Do not trust the first number after deleting. macOS recalculates storage lazily and snapshot removal in particular can take several minutes to appear.

Realistic expectations

The quick wins list recovers somewhere between five and fifty gigabytes on a Mac that has not been cleaned, and the spread is almost entirely down to Downloads and old device backups. Snapshot removal adds anything from ten to a hundred and fifty gigabytes, which is why cleaner apps advertise such impressive numbers.

The honest limit is that a 128GB or 256GB Mac with a real photo library and a couple of large apps will keep filling up. No cleaning routine changes that arithmetic. At that point the choices are iCloud optimisation, an external drive for archives, or a larger machine — and the first is cheapest.