# "Your startup disk is almost full" on a Mac: what to do

> The macOS warning appears only when a few gigabytes are left, which is late. Here is the order to work in, the four folders that are usually the whole answer, and what to do if the disk is already too full to delete anything.

Canonical: https://bytesweep.app/blog/mac-startup-disk-full.html
Last updated: 2026-10-16

## “Your startup disk is almost full” — what to do in the first ten minutes

macOS warns late. The “Your startup disk is almost full” dialog appears only when the volume is down to a few gigabytes of truly free space, and by then Safari may be refusing to save downloads and Xcode failing builds with errors that don’t look like a disk problem at all. On most Macs the fix is four folders, and none of them hold your documents.

- By [Shajeen Ahamed](https://bytesweep.app/about.html#author)
- Published 25 September 2026
- Last updated 16 October 2026
- 4 min read
- Part of [How to free up space on a Mac](https://bytesweep.app/free-up-space-on-mac.html)
- [How we write](https://bytesweep.app/about.html#how-we-write)

### Two minutes before you delete anything

1. Get the real number. The Finder folds purgeable space into “available”, which flatters you. `df -H /System/Volumes/Data` shows what’s actually free, and that’s the figure that matters right now.
2. Quit the apps that hold big files open: Photos, Mail, Docker Desktop, Xcode, any virtual machine. A file deleted while an app still has it open keeps its space until that app exits, which is the usual reason a clean-up “does nothing”.
3. Leave the Trash alone for now. If you delete the wrong thing in the next ten minutes, the Trash is your only undo. Empty it at the end.

Never delete inside `~/Library` by the folderful. Next to the caches you can throw away sit your mail, your keychains, your licences and your local databases. Every step below names a specific path for that reason. [What is safe to delete in the Library folder](https://bytesweep.app/answers/mac-library-folder-what-to-delete.html) goes through it folder by folder.

### The four places the space usually is

On a Mac in daily use for two or three years, four things account for most of the missing space. Check all four before you so much as open Photos.

*Typical ranges, not measurements. Any one of the four can outweigh everything else on the disk, and the browser cache never does.*

Time Machine’s local snapshots come first. If Time Machine is set up, macOS takes an hourly snapshot of the internal drive whether or not the backup disk is connected, and together they often hold 10 to 80 GB. They’re counted as purgeable rather than used, which is why the numbers never agree. List them:

```
tmutil listlocalsnapshots /
```

For “my disk is full and I can’t see why”, this is the most common answer. [How local snapshots work, and how to thin them safely](https://bytesweep.app/blog/time-machine-local-snapshots.html) goes through it properly.

If you write software, `~/Library/Developer/Xcode/DerivedData` is usually the largest directory in your home folder, and all of it is disposable build output. See [what DerivedData actually is](https://bytesweep.app/blog/deriveddata-explained.html).

Docker keeps everything in one file, `Docker.raw`, that grows with every image you pull and doesn’t shrink when you delete one. Don’t delete the file itself: it contains every image, container and volume you have. [Shrinking Docker.raw without losing anything](https://bytesweep.app/blog/docker-raw-shrink.html) covers the safe route.

Local AI models grow faster than anything else on a developer’s Mac, and mainstream cleaners don’t look for them. Ollama, LM Studio and the Hugging Face cache each keep their own store, and any one of them can pass 100 GB. Start with [where Ollama keeps its models](https://bytesweep.app/blog/ollama-models-disk.html).

### Measure them in one go

The Finder calculates folder sizes lazily and often gets them wrong on a busy disk. `du` doesn’t:

```
du -sh ~/Library/Developer/Xcode/DerivedData \
      ~/Library/Containers/com.docker.docker \
      ~/.ollama ~/.cache/huggingface \
      ~/Library/Application\ Support/MobileSync/Backup \
      ~/Downloads 2>/dev/null | sort -h
```

Anything missing from the output isn’t installed. Work from the bottom of the sorted list up. One 40 GB directory is worth more than forty 1 GB ones, and it’s far less likely to include something you wanted. If none of these is the culprit, [find the large files](https://bytesweep.app/answers/find-large-files-on-mac.html) and then [check folder sizes](https://bytesweep.app/answers/see-folder-sizes-on-mac.html), because a folder of many small files never shows up in a search by size.

`~/Library/Caches` isn’t in that list on purpose. Most of it is safe to clear, but it’s usually a few gigabytes that come straight back, and a few apps keep things there that are slow to rebuild, like a mail index or a large thumbnail database.

### When the disk is too full to delete

Below about 2 GB free, even a delete can fail. APFS is copy-on-write, so removing a file means writing new metadata first, and moving it to the Trash is a move, not a removal. Two ways out:

- Delete one large file directly with `rm` in Terminal, skipping the Trash. One Docker image or a DerivedData folder buys back enough room to work.
- Or restart. That clears swap and temporary files, and macOS releases purgeable space readily when it’s under real pressure. It usually returns a few gigabytes on its own.

If the Mac won’t boot at all, that’s a different procedure: recovering a Mac that will not start because the disk is full.

### Not worth your ten minutes

| Commonly suggested | What it actually returns |
| --- | --- |
| Clearing Safari’s cache | Usually under 1 GB, and it rebuilds within a day |
| Deleting “Other” or “System Data” | Not a folder; it can’t be deleted as a unit. See [why that number never adds up](https://bytesweep.app/blog/system-data-explained-numbers.html) |
| Reinstalling macOS | Hours of work, and it keeps your home folder, which is where the space went |
| Deleting language files from apps | A few hundred megabytes, and it breaks code signatures |

If the warning shows up while the Finder still claims 20 GB available, much of that 20 GB is purgeable. macOS only releases it under real pressure, so you can see “20 GB available” and still fail to save a file.

Freeing space makes the Mac faster only if you were near full. Below roughly ten per cent free, macOS runs short of room for swap and temporary files and slows down. Above that, more free space buys nothing.

Expect to repeat this every few months if you do it by hand. DerivedData returns with the next build, snapshots within the hour, and Docker with the next image you pull.

### Let Bytesweep clear this for you

Skip the commands. Bytesweep finds this for you, rates every item safe, review first or keep, and clears what you choose. Most of what it removes can be put back for seven days. See what it finds on your own disk, free for 3 days, then pay once with no subscription.

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Related in Bytesweep: [Junk cleaner](https://bytesweep.app/junk-cleaner.html)

### Keep reading

- [**Purgeable space on a Mac, and how to force it back** Purgeable is space macOS believes it can reclaim but has not. What goes into it, why the Finder counts it as available when nothing else does, and the reliable ways to make macOS actually release it.](https://bytesweep.app/blog/purgeable-space-mac.html)
- [**Time Machine keeps snapshots on your Mac, not just the backup drive** macOS takes hourly snapshots of your internal disk whether or not a backup drive is attached, and they commonly hold 10 to 80 GB. How to list them, how to thin them, and which one you must not delete.](https://bytesweep.app/blog/time-machine-local-snapshots.html)
- [**You emptied the Trash and got nothing back: here is why** Four separate mechanisms stop a deleted file from returning its space on macOS, and only one of them is the Trash. How to tell which one you have hit, and what to do about each.](https://bytesweep.app/blog/empty-trash-mac-space.html)