Chapter 07Built-in tools only
Time Machine and the snapshots on your internal disk
Even without a backup drive attached, macOS records hourly snapshots on your SSD. Understanding them explains where some System Data goes and when it is safe to reclaim.
This is the chapter that confuses people most, because the numbers seem to contradict each other: Finder reports plenty of free space while a large installation still refuses to start. The answer is usually a local snapshot, and the answer is usually good news.
What a local snapshot is
APFS, the file system used by modern Macs, can take an instant point-in-time picture of your data without copying everything. Because unchanged files are shared between the snapshot and the live system, a snapshot costs almost nothing until files change, at which point the old versions are preserved inside it. Time Machine creates one roughly every hour and keeps them for about a day. Apple calls the space they occupy purgeable, because macOS will thin old snapshots automatically when an app needs room.
Why free space looks wrong
| What you see | What it means |
|---|---|
| Finder says 40 GB free | Includes purgeable space: old snapshots, caches and iCloud placeholders |
| System Settings says 12 GB free | Counts only space that is immediately usable without purging |
| An installer refuses to run | It needs contiguous space that macOS has not yet reclaimed |
Nothing is broken. The system is simply holding recovery data until it is genuinely needed. Giving it a minute, or restarting, often resolves the contradiction.
Inspect snapshots with built-in tools
You do not need to install anything. Open Terminal, which ships with macOS, and list the snapshots on the data volume:
tmutil listlocalsnapshots /prints every local snapshot with its timestamp.tmutil thinlocalsnapshots / 100000000000 4asks the system to thin snapshots until about 100 GB is free, working from oldest to newest.
Run the first command to see what is stored; run the second only when you have verified that a real backup exists elsewhere. The figures are in bytes, which is why the second command looks like a very large number.
A snapshot on the same disk cannot protect you from a failed SSD, a lost laptop or ransomware. It protects against the mistake you made an hour ago. Keep the distinction clear in your head before you thin anything.
Attach a backup drive and let Time Machine work
The healthier fix is to connect an external drive and let Time Machine maintain real backups there. Once an external backup exists, macOS is far more willing to release local snapshots, and the pressure on the internal SSD eases on its own. Choose System Settings > General > Time Machine > Add Backup Disk, then let the first backup finish overnight.
When to keep snapshots
- Keep them when you are editing documents you might need to roll back within a day.
- Keep them when your only other copy is in the cloud and you are about to travel without reliable internet.
- Keep at least one snapshot when you are about to make risky changes, such as migrating a large library.
When thinning is reasonable
- An external Time Machine backup completed within the last day.
- You are low on space and have no imminent rollback need.
- You are installing a large update and the installer needs immediate free room.
Treat snapshots as a safety net with a short memory, not as permanent storage. Thinned deliberately, they give back meaningful space; left alone, macOS will manage them on your behalf.
Prefer the whole method in one file?
The Mactria Help e-book collects every chapter of this guide into a single, printable PDF with checklists, safe-deletion rules and a 30-minute cleanup routine.