Even if your Time Machine backup drive is unplugged, macOS still saves hourly snapshots on the internal disk. That hidden layer can grow to 50-100 GB and show up as "System Data" or "Other."

Short answer: clear local Time Machine snapshots with tmutil in Terminal. The command is built for this and does not affect backups already stored on your external drive.

Why local snapshots exist

Time Machine keeps an hourly local snapshot of changed files so you can still restore recent edits even when the external backup drive is not attached.

macOS marks these snapshots as "Purgeable" and promises to delete them when space runs low. In practice, that cleanup often fires too late — big installs or partition operations fail because the system has not caught up.

Clearing local snapshots with tmutil

Terminal sounds intimidating, but tmutil is designed for exactly this cleanup.

  1. open Terminal (Applications > Utilities, or via Spotlight)
  2. list the current snapshots: tmutil listlocalsnapshots /
  3. to free around 10 GB urgently, run: tmutil thinlocalsnapshots / 10000000000 4
  4. to delete one specific snapshot, run: sudo tmutil deletelocalsnapshots 2025-10-15-142031 and enter your password

The date in the last command has to match one from the list in step 2.

When the weight is not snapshots

If Terminal shows only a handful of snapshots but "System Data" is still huge, the weight is usually in app caches, Xcode derived data, or log files — not Time Machine.

Better next routes

If you still cannot find the source of the bloat, continue with How to Find Large Hidden Files on Mac.

For the broader "what is Purgeable" framing, use What Is Purgeable Storage on Mac?.