Time Machine is best known for the external drive it fills, but it has a quieter habit: it also keeps short-lived copies on your internal drive, called local snapshots. They appear in the System Data group, they can be large, and they are easy to mistake for waste. They are not waste. They are the reason a document you regret changing can be recovered when no external drive is plugged in.
What a local snapshot is
A snapshot is a record of how your files looked at a moment in time. macOS takes one roughly every hour while a drive is connected, and keeps them for about a day before making room for newer ones. Because snapshots share unchanged data, a stack of them is far smaller than a set of full copies would be. Still, on a busy machine with large files, the stack can add up to a noticeable slice of the drive.
Why they exist
Snapshots give you a way back without asking you to plan ahead. If a file is edited, corrupted or removed and you had no external backup handy, a snapshot from earlier in the day may still hold the earlier version. That is a real safety net, and it is why the chapter begins by defending them rather than dismissing them. The goal is understanding, not removal for its own sake.
See what they hold
- Open System SettingsClick the Apple menu and open System Settings.
- Go to General, then Time MachineChoose General in the sidebar and then Time Machine.
- Read the snapshot informationThe panel describes the local copies and when the next one is due. Some versions show an estimate of the room they hold.
- Compare with StorageOpen the Storage panel and note how the System Data group changes as snapshots age out over a day.
- Decide, do not guessIf you rely on snapshots for safety, keep them. If you keep full external backups and need room now, letting the current stack settle is reasonable.
When to keep them, when to let them go
Keep local snapshots whenever your only recent safety copy lives on this Mac. That is common for people who back up to an external drive only occasionally, or who work between backups. In that case the snapshots are quietly protecting a day of work you might otherwise lose.
It is reasonable to let them settle when you already hold a current full backup on an external drive, and the drive is under pressure right now. Even then, the choice is not permanent: clear the current stack and the system simply builds a new one over the following hours. Nothing about snapshots is destructive to your live files.
| Situation | Sensible approach |
|---|---|
| No recent external backup | Keep snapshots; they are your nearest safety copy |
| Current external backup exists | Let the stack settle if room is needed now |
| A file was changed by mistake | Restore from a snapshot before it ages out |
| Drive repeatedly full | Move the main backup to an external drive and review large files |
Snapshots and your external backup serve different jobs. The external copy is the long-term safety net; snapshots are the short-term one. A healthy Mac has the long-term copy, and treats the short-term copies as a bonus rather than a burden.
Backups, briefly
Device backups and full backups are the other half of this group, and they deserve the same respect. A backup is the single most valuable thing on your drive, so it should never be the thing you remove in a hurry. If your drive is small, the better move is usually to keep backups on an external drive and leave the internal one to carry live work. That one decision resolves more storage pressure than any amount of tidying ever will.
Read this chapter alongside the one on System Data. Between them they cover the two parts of the disk where caution matters most, and where a little understanding goes a long way.