Local snapshots of Time Machine is a feature of macOS that serves as a temporary backup solution, until your Mac has a chance to connect to its permanent Time Machine disk, such as Time Capsule.
In macOS High Sierra (10.14) and later, Time Machine stores snapshots on every APFS-formatted, all-flash storage device in your Mac or directly connected to your Mac. In earlier macOS versions, Time Machine stores snapshots on the internal startup disk of your Mac notebook computer and any Mac OS Extended (HFS Plus)-formatted storage devices directly connected to that computer.
The snapshots can take up a significant amount of disk space (up to 80% of the disk), This peculiarity often causes confusion to Mac users, because it appears like the System category is taking too much space on macOS High Sierra and later, in the About This Mac window.
On macOS High Sierra and later with APFS, the snapshots are located on your startup disk, but beyond any scannable area, i.e. their inner contents are not accessible for the user.
On older macOS versions, the local snapshots are located inside the restricted folder at /.MobileBackups. You can reveal its content by scanning as administrator. Note that DaisyDisk will still include the size of snapshots into the purgeable space.
DaisyDisk shows the snapshots in the special snapshots item, a child of the hidden space. You can inspect the list of the snapshots, their estimated sizes and also delete them.
This feature is partially limited in the Mac App Store edition of DaisyDisk. In particular, snapshot size estimation is not available and to delete snapshots you’ll have to use Terminal. More about DaisyDisk versions.
Each APFS snapshot only stores changes in the file system, and the rest of unchanged data blocks on disk are simply linked to. For this reason snapshots appear quite small, and can only get big after you make big changes, for example, delete a lot of big files.
Also, snapshots share data blocks between each other, so that when estimated as a group, snapshots’ size on disk will not be equal to the sum of all snapshots taken separately. Also after you delete one snapshot, the size of other snapshots may appear to have increased, because the shared blocks of the deleted snapshot may now be counted toward other snapshots.
Also note that the purgeable space already includes snapshots, as a rule. For this reason, in DaisyDisk, the snapshots item does not contribute to the total size of hidden space, and snapshots’ sizes are displayed only for reference.
Most users will encounter only snapshots created by Time Machine (i.e. macOS). However, there are third-party apps, particularly the popular Carbon Copy Cloner by Bombich, that are capable of creating their own snapshots. Such third-party snapshots are not counted toward the “purgeable space” and not managed by macOS. Such snapshots can be deleted only by the app that has created them. This is another reason why the snapshots item in DaisyDisk is useful: it can make you aware of the third-party snapshots, which could otherwise silently consume your disk space and not get automatically removed — quite an often encountered issue for Mac users.
It usually doesn’t make much sense to delete the snapshots, because just after you purge them, new snapshots will immediately begin to build up again. Instead, you can just let macOS take care of the snapshots automatically:
Snapshots older than 24 hours are automatically deleted. And to make sure that you have storage space when you need it, snapshots are stored only on disks that have plenty of free space. When storage space gets low, additional snapshots are deleted, starting with the oldest. That’s why Finder and Get Info windows don’t include local snapshots in their calculations of the storage space available on a disk.
However, in certain cases it may be desirable to forcedly delete the local snapshots. You can do it in DaisyDisk by dragging the snapshots item (or any of its child snapshot items), or alternatively the purgeable space item to the Collector, and delete as you’d do with a regular file or folder.
This feature is partially limited in the Mac App Store edition of DaisyDisk. To delete snapshots you’ll have to copy the script provided by DaisyDisk, paste it in Terminal, and press Enter to execute. Then rescan the list of snapshots in DaisyDisk. More about DaisyDisk versions.
You can also use the command-line utility tmutil to manage the local snapshots manually.