Time Machine local snapshots: the hidden GB behind “System Data”

Your Mac can keep Time Machine local snapshots on its internal disk even if you've never plugged in a backup drive, and even if you don't think you use Time Machine at all. They're point-in-time copies of your filesystem that quietly hold onto old file blocks, which is how they inflate "System Data" and make your free-space number stop adding up. The honest part is that macOS usually manages them for you, so most of the time you shouldn't touch them.

Where Time Machine local snapshots come from

There are two everyday sources, and neither requires a backup drive to be connected.

• Time Machine's hourly stopgap. When Time Machine is turned on, it takes an hourly snapshot and keeps a copy on your internal disk, not only on the external drive. That's by design: if the backup drive isn't attached, the local snapshot preserves your recent history until the drive comes back. So the snapshots live on your Mac, and they keep accumulating while the drive is unplugged.
• The pre-update snapshot. macOS itself takes a snapshot before installing a system update, so it can roll back cleanly if the update fails. This happens regardless of Time Machine.

That's why "I don't use Time Machine" so often still means "I have local snapshots." Either Time Machine is quietly on to a drive you rarely connect, or macOS made one around an update.

Why they inflate "System Data"

A snapshot is cheap the instant it's taken. APFS copies only the filesystem metadata; nothing is duplicated at that moment. The cost comes afterward: as you change or delete files, their original blocks have to be retained for as long as a snapshot still references them. That quiet retention is the space.

Because snapshots share blocks, a group of them is smaller than the sum of each measured alone, and a snapshot of a disk that hasn't changed costs almost nothing. But over a day of edits, the retained blocks add up, and in Storage settings that weight lands in the residual System Data bucket. It's a big reason the free-space number you see doesn't match what you think you deleted.

One honest caveat: whether snapshots show up as purgeable specifically is disputed. Independent analysis finds macOS often counts them as used space instead. The full breakdown is in what purgeable space actually is. Either way, the effect on your free-space reading is the same: real space, held by something you didn't knowingly create.

macOS already manages them (so usually, do nothing)

This is the part the cleaner apps skip. macOS thins local snapshots on its own:

• Snapshots older than 24 hours are automatically deleted.
• They're only kept on disks that have plenty of free space to spare.
• When storage runs low, macOS deletes more of them, starting with the oldest.

So on a healthy disk, local snapshots are self-limiting. They're not a leak. They're a rolling ~24-hour safety net that macOS trims for you. Manually deleting them rarely helps, because if your disk were actually under pressure, macOS would already be clearing them. If your disk is jammed, the culprit is usually real files elsewhere (see why your storage is full).

See them without the Terminal

If you just want to look, Disk Utility can list them:

1. Open Disk Utility (Applications → Utilities).
2. In the menu bar, choose View → Show APFS Snapshots.
3. Select your volume (usually "Macintosh HD" or its Data volume) in the sidebar.

You'll see the snapshots listed by date. You can remove one with the delete (–) button, but note that Disk Utility won't give you a trustworthy "this is how much you'll get back" number, because, as above, exactly how snapshots are accounted for is murky. Deleting one is real, though: it removes that restore point permanently.

The GUI lever: turn Time Machine off

If your Mac keeps making snapshots and you'd rather it didn't, the clean, reversible switch is Time Machine itself:

Apple menu → System Settings → General → Time Machine, then select your backup disk and remove it.

With no backup destination, macOS stops taking the hourly local snapshots. The trade-off is real and worth stating plainly: you also give up Time Machine's local restore points, so a fat-fingered delete is no longer recoverable from an on-disk snapshot. macOS may still create a brief snapshot around a system update, but it cleans those up on its own.

Clear them from the Terminal (power users)

macOS ships a command-line tool, tmutil, if you'd rather see and thin snapshots directly. List what's currently on your boot volume:

tmutil listlocalsnapshots /

Delete all local snapshots on that volume:

tmutil deletelocalsnapshots /

Or thin snapshots just enough to reclaim a target number of bytes; the trailing 4 is the urgency level (1–4, where 4 is most aggressive):

tmutil thinlocalsnapshots / 5000000000 4

Three honest caveats before you run any of these:

• macOS normally handles this. On a disk with headroom, these commands mostly delete snapshots that would have expired within a day anyway.
• You're deleting restore points. Every snapshot removed is a local recovery option gone. If you delete a file and then delete the snapshots, there's no rolling back to it.
• It won't fix a full disk on its own. If the disk is truly out of room, macOS was already thinning snapshots; the durable fix is finding the real files, not the snapshots.

The honest bottom line

Local snapshots are macOS keeping a short-term safety net on your own disk: created by Time Machine and by the updater, trimmed automatically, and disputed enough in the accounting that even experts can't agree whether they count as "purgeable." You can watch them (Disk Utility, or tmutil listlocalsnapshots /), you can stop them (turn Time Machine off), and you can clear them by hand (tmutil deletelocalsnapshots /), but in almost every case the right move is to leave them to macOS and go looking for real, reclaimable files instead.

Telling "space macOS will free on its own" apart from "space that's actually yours to clear" by hand means cross-reading Disk Utility, tmutil, and Finder and hoping you trust the right number. Plumby surfaces snapshot-held space plainly. It shows how much is tied up in local snapshots versus what's reclaimable, names the exact paths, and never pretends it can delete something macOS controls. It won't fight the system for a gigabyte macOS would have freed anyway. It just shows you where the space that is yours to reclaim actually sits.