Softwaremacmacos-tahoetime-machineapfsstoragetmutil
Deleting big files on a Mac can free no space for half an hour, because Time Machine's hidden snapshots still hold it
Howard Oakley found macOS Tahoe reporting 0 bytes purgeable instead of 94.9 GB after deleting files, and one volume took 29 minutes to show the space. Here's how to see the snapshots and clear them yourself.
Apple’s position is that you shouldn’t have to think about this. Its support page on local snapshots says Time Machine saves one snapshot of the startup disk “approximately every hour, and keeps it for 24 hours,” and “Your Mac counts the space used by snapshots as available storage.” In theory, macOS purges snapshots the moment you need the room.
Oakley built a test to see if it does. On an external SSD he filled a 100 GB container with large files, let Time Machine snapshot it, then deleted the files. When he tried to copy a big file in, the copy was refused. The system’s purge service asked Time Machine how much it could free and got “0 bytes instead of the 94.9 GB that should have been purgeable,” 24 minutes after the deletion. One volume finally showed its purgeable space 29 minutes after deletion; another still showed none after 36.
Oakley’s earlier piece notes he first reported inconsistent purgeable-space accounting “over eight years ago.” Problems that survive that long tend to be designed-in tradeoffs rather than bugs anyone is chasing, which makes knowing the manual fix worth more than waiting for a patch. For a forced purge, the tmutil manual also offers thinlocalsnapshots, which takes a mount point, a byte count and an urgency from 1 to 4.
Oakley’s tests ran on an external SSD; he didn’t say whether the startup disk behaves the same way.