Time Machine snapshots and System Data
Updated · Dmytro Virych
Local copies macOS keeps between backups. It makes new ones anyway. Apple's documentation calls them local snapshots, and they are part of the grey System Data block.
What a Time Machine snapshot is
When your backup disk is not connected, Time Machine still wants to be able to restore the file you changed an hour ago. So it keeps a snapshot of your startup disk: a frozen picture of every file as it was at that moment, stored on the same disk. When the backup disk comes back, the snapshot is copied across and the local one is no longer needed.
A snapshot is not a second copy of your whole disk. It shares every unchanged block with the live volume and only holds on to blocks that have since changed or been deleted. That is why a fresh snapshot costs almost nothing and an old one, on a disk you have been working on, can hold a great deal.
Why it counts as System Data
Apple's storage bar has no segment for snapshots, so their space lands in the grey block. It is also why the block can grow after you delete something large: the file left your folder, but the snapshot taken before you deleted it still holds its blocks until the snapshot itself goes.
macOS does remove these on its own when the disk comes under pressure. It does not always do so before the storage warning appears, and it does not tell you which ones it kept.
How to see them
Open Terminal and ask Time Machine for its snapshot dates on the startup disk:
tmutil listlocalsnapshotdates /
Each line is one snapshot, named by the moment it was taken. This command lists only Time Machine’s own snapshots, which is the point of using it rather than the broader listing below. If it prints nothing but a heading, there are none to remove.
tmutil listlocalsnapshots /
This one lists every snapshot on the disk, including the ones macOS keeps for itself. Those have names beginning com.apple.os.update. They are not Time Machine’s and the next section is about leaving them alone.
The snapshots to leave alone
macOS creates its own snapshots around system updates, named com.apple.os.update-…. One of them is typically what the system would roll back to if an update failed, and one may be marked as limiting how small the disk’s container can shrink. Removing one is system surgery, not freeing up space, and it does not shrink System Data in any way you would want.
The safeguard is the command above: tmutil listlocalsnapshotdates returns Time Machine’s dates and nothing else, so if you only ever delete dates that command printed, you cannot touch a system snapshot by accident. MacMemory applies the same rule and a second one on top: it offers a snapshot only if Time Machine lists it and the file system marks it as one macOS may discard.
Why no tool can tell you the size of one snapshot
You will look for a size column and not find one, in Terminal or in any app. That is not a gap in the tools. A snapshot shares blocks with the live disk and with every other snapshot, so "the size of this snapshot" is only well defined as the blocks unique to it, and that number changes the moment any other snapshot is created or removed.
What is well defined is the total that would come back if all of them went, and that is the figure a scan can report for the category as a whole. Any tool that prints a confident number beside each snapshot is estimating.
How MacMemory handles them
Snapshots are listed under Time Machine snapshots with the label Safe to delete, by date and age, with one total for the category and no size per row, for the reason above. The row says, as soon as you tick it, that these are removed outright rather than moved to the Trash: it is one of four exceptions to the rule that everything the app deletes goes to the Trash (the Trash itself, Docker images and unused simulators are the others), and the app states the exception where you can see it.
The detail screen carries one sentence, and it is the same one this page relies on:
Time Machine will create new snapshots on its own. Removing these does not affect backups on your backup disk.
How to remove Time Machine snapshots by hand
Three commands in Terminal, on macOS 14 and later. No administrator password is asked for: the snapshots belong to your account already.
List Time Machine's snapshots. Safe to delete
Print the dates of the Time Machine snapshots on your startup disk. Only these dates are candidates for the next step.
tmutil listlocalsnapshotdates /Undo: Nothing is changed by listing.
Delete one snapshot by its date. Safe to delete
Copy a date from the list exactly as printed and pass it to the delete command. macOS answers with the name of the snapshot it removed. Repeat for each date you want gone, or leave the newest one so a very recent change can still be restored.
tmutil deletelocalsnapshots 2026-09-01-120000Undo: None. The snapshot is removed immediately. Time Machine will create new snapshots on its own, and the backups on your backup disk are unaffected.
Check the result. Safe to delete
Run the listing again to confirm the date is gone, then look at System Settings, General, Storage. The bar can take a minute to catch up.
tmutil listlocalsnapshotdates /Undo: Nothing is changed by listing.
The shortcut
MacMemory does the steps above from one screen. It scans for free, lists what it finds with one plain sentence per row and the same two labels — Safe to delete and You decide — and moves what you tick to the Trash. Four exceptions are stated on the row that offers them: Time Machine snapshots, the Trash itself, Docker images, which only Docker can remove, and unused simulators, which only Xcode can erase, are removed outright. Mail, backups, snapshots and the Trash need Full Disk Access to be read at all, and a category it cannot read shows a lock rather than a zero.
Nothing it deletes is beyond what this page describes. What it saves you is the Terminal. What it costs · Questions people ask