Time Machine Local Snapshots Explained: Why Mac Space Disappears
Time Machine local snapshots are hidden, read-only APFS volume states created automatically on your internal Mac SSD when an external backup drive is unplugged. Because they preserve older versions of files, newly deleted files remain frozen on disk and continue occupying space until snapshots are thinned by macOS or via tmutil.
Have you ever deleted a 50 GB folder on your Mac, emptied the Trash, and noticed that your available disk space didn't increase by a single gigabyte? You are dealing with APFS Local Snapshots.
How APFS Snapshots Work
The Apple File System (APFS) uses a copy-on-write architecture. When Time Machine creates a local snapshot, it freezes the filesystem state at that moment in time. If you later "delete" a 20 GB video file, macOS simply unlinks the pointer from Finder, but keeps the underlying data blocks locked on your SSD because the snapshot requires them!
Listing and Thinning Snapshots via Terminal
Open Terminal.app and run:
Output example:
To ask macOS to thin out 20 GB of local snapshots safely:
The parameter 4 represents the highest urgency level, instructing APFS to release locked sectors immediately.
Automate Safe Offloading with Nebula
Nebula identifies files you haven't opened in months, streams them safely with SHA-256 validation to your external SSD, NAS, or Cloud, and lets you restore in one click.
Join the Mac Waitlist — Founding Offer ($9.99)