Disk Utility is not a storage tool, and that confuses everyone
It repairs volumes, shows containers and formats drives. It does not tell you what is using space, and here is what to use for each job.
Disk Utility looks like it should tell you what is using your disk, and it never does. It is a tool for volumes rather than files, and once that is clear it stops being frustrating and starts being useful for the things it is genuinely for.
What it is for
- Formatting and partitioning. Preparing an external drive, choosing APFS or exFAT.
- First Aid. Checking and repairing the file system structure of a volume.
- Seeing the container. How much of an APFS container each volume is using, including snapshots.
- Mounting and ejecting. Including images and volumes Finder is not showing.
None of those is about which folder is large, which is why it feels like the wrong tool when you arrive at it from a full disk.
Why its numbers disagree with Finder
Disk Utility reports at the container and volume level, which includes snapshots and the system volume, while Finder reports available space including purgeable. The two answer different questions and both are correct.
The gap between them is usually local Time Machine snapshots. What purgeable space is explains that number, and what the other volumes in your container are explains the rest of the layout, including why the system appears as a separate volume.
When First Aid is the right answer
There is one storage situation where Disk Utility genuinely helps: when the numbers make no sense at all, files behave strangely, or space is reported as used with nothing accounting for it even after a proper look. A damaged file system can misreport space, and First Aid checks for exactly that.
Run it from Disk Utility on the data volume, and from Recovery if it reports problems it cannot fix while the volume is in use. It is safe to run and it changes nothing when the volume is healthy.
The right tool per question
| Question | Tool |
|---|---|
| Which folder is using 40 GB | A disk map, or du in Terminal |
| Is the file system damaged | Disk Utility, First Aid |
| How is my container laid out | Disk Utility |
| What is in System Data | Neither. See the guide |
| Format a drive for a Mac | Disk Utility, choose APFS |
For the first row, the macOS storage screen and what it hides covers the built in option, and the disk is full with nothing to delete covers the case where nothing visible accounts for the space.
Common questions
Does Disk Utility show what is using disk space?
No. It works at the volume and container level, showing how volumes share space, but it never lists folders or files. For that you need the storage screen in System Settings, a disk map, or du in Terminal.
Why does Disk Utility show a different size than Finder?
Because they measure different things. Disk Utility reports container and volume usage including snapshots, while Finder reports available space including purgeable. The difference is usually local Time Machine snapshots.
Does running First Aid free up disk space?
Not normally. It checks and repairs the file system, and only recovers space in the specific case where damage was causing space to be misreported. It is safe to run and does nothing on a healthy volume.
What format should I use for an external drive on a Mac?
APFS if the drive will only be used with Macs, since it supports the metadata macOS relies on. exFAT if you need to share it with Windows, accepting that it handles Mac metadata poorly.