Your Mac's storage was fine yesterday, and today it is nearly full, and you cannot point to anything you downloaded or created that would explain it. Before you go looking further, run tmutil listlocalsnapshots / in Terminal. There is a good chance the answer is Time Machine's local snapshots — hourly, automatic backups-of-a-sort that macOS takes on your internal disk whether or not any backup drive is attached, and normally keeps for roughly a day. This article explains that specific behaviour: the hourly cadence, the rough 24-hour retention window, why it sometimes lingers far longer than that, and the exact steps to check and clear it.

Time Machine snapshots your internal disk on its own schedule

Most people assume Time Machine only does anything when its backup disk is plugged in. It does not work that way. As long as Time Machine is turned on in System Settings, macOS takes a local snapshot of your startup volume roughly once an hour, entirely independent of whether the external drive or network destination is connected. This is what lets you browse recent file versions through Time Machine's interface even on a laptop that has been away from its backup disk for days — the local snapshots are standing in for the real backup in the meantime.

The mechanism is the same APFS snapshot technology used everywhere else on the system: cheap to create, because nothing is copied at the moment it is taken, but capable of holding onto increasing amounts of data as files change underneath it.

The roughly-24-hour retention window, and when it doesn't apply

Under normal conditions, Time Machine keeps local snapshots for about a day and then age them out on its own, replacing old ones as new ones are taken. That is the behaviour most Macs exhibit most of the time, and it is why local snapshots are usually invisible: they turn over fast enough that they never accumulate.

Two situations break that pattern:

  • The backup destination is missing or unreachable for an extended period. If your Time Machine disk has not been connected in weeks, or a network backup destination is offline, macOS has less pressure to prune aggressively, and local snapshots can persist well past the usual window.
  • The destination volume is full. If the actual Time Machine backup is failing because its own disk has no room, the local snapshots that were meant to bridge until the next successful backup have nowhere to hand off to, and they can pile up on your internal disk instead.

Either way, the result is the same: snapshots that should have expired in a day are still sitting there weeks later, each one pinning down blocks that would otherwise be free.

The specific symptom: a disk that looks full with no obvious cause

This is the pattern to recognise: you have not installed anything large, your Downloads folder is unremarkable, and yet Available space in About This Mac → Storage Settings has dropped noticeably, or the "used" figure has crept up over days without a matching increase in anything you can see in Finder. Nothing in the Storage panel's category breakdown points at Time Machine directly — this space typically shows up folded into the general "System Data" bucket, unlabelled. That combination — falling free space, nothing to blame in Finder, a general mystery category growing — is the signature of local snapshots that have overstayed their welcome.

Checking what's actually there

The check costs nothing and takes a few seconds:

tmutil listlocalsnapshots /

If Time Machine's snapshots are behaving normally, you will see a small number of recent entries, each an hour or so apart. If you see a long list spanning days or weeks, or a handful of very old dates alongside recent ones, that is the confirmation you are looking for.

Deleting stubborn snapshots

Local snapshots that have outlived their usefulness can be removed directly:

sudo tmutil deletelocalsnapshots 2026-08-02-140000

Substitute the exact date string from listlocalsnapshots for the one shown above — do not invent one. This needs sudo because it modifies the boot volume's snapshot state. Deleting a snapshot only removes that one local recovery point; it has no effect on any backup you already have on an external drive, and macOS will keep taking fresh hourly snapshots afterward as normal.

If several old snapshots are involved, delete them from oldest to newest. Because snapshots can share references to the same underlying blocks, freeing the oldest one first is usually what actually returns space, rather than the newest.

Reconnecting the real backup disk clears the backlog properly

The more reliable fix, if you have a Time Machine disk you have simply not connected in a while, is to plug it back in and let a backup complete. Once Time Machine finishes writing to the actual destination, it reconciles the local snapshots against what has now been safely backed up and prunes what it no longer needs to keep locally. This is the mechanism working as designed — local snapshots exist to cover the gap until the next real backup, and a completed backup is exactly what closes that gap.

Why purgeable space doesn't shrink the moment you delete files

This same mechanism explains a related and confusing symptom: you delete a large file, empty the Trash, and the "Used" or "Purgeable" figure in the Storage panel barely moves. If a local snapshot was taken before you deleted the file, the snapshot still references the blocks that file used to occupy, and APFS cannot reuse them until every snapshot pinning them is gone. The space is accounted for correctly by macOS — it is purgeable, meaning reclaimable in principle — but it will not visibly free up until the relevant snapshots age out or are deleted, or until macOS decides it needs the room and reclaims it itself.

Conclusion / VolumeLens tie-in

Time Machine's local snapshots are a genuinely useful feature working exactly as intended almost all of the time, which is precisely why they are so easy to misdiagnose the rare times they misbehave. A disk that looks suddenly full, with no matching activity in Finder, is worth ten seconds with tmutil listlocalsnapshots / before you start deleting things you actually want.

We built VolumeLens because we needed the story a Mac tells about its own storage to be honest, fast, and safe, rather than a single opaque number you have to take on faith. VolumeLens scans the real folders you give it access to and shows their true sizes — hard links counted once, sparse and compressed files flagged, anything it could not read labelled rather than hidden — so that when your free space and your file sizes stop adding up, you know the mismatch is something else, like a snapshot, rather than a scanning tool getting the numbers wrong. Have a look at what VolumeLens actually checks before you go looking for the difference yourself.