A couple of days ago, I wrote about what I still consider to be a gross error in the Finder’s reporting of disk free space. You’ll recall the screenshot I showed at the time, reporting “83.71 GB purgeable”. This article considers just what counts as being “purgeable”.

At the time, my own free utility Mints reported quite different figures, for slightly different variables.

The important values there are:

• available for important usage 144.12 GB
• available for opportunistic usage 131.77 GB

which don’t say anything about a loose 83.71 GB, nor 227.83 GB of available disk space.

Apple defines those metrics separately. For the purgeable space given in the Get Info dialog and Disk Utility, it writes that “purgeable space” is “space that macOS can free up when needed by removing files from your computer (you can’t manually remove the files that are designated purgeable, but macOS removes them as space is required)”.

Many users across many discussions and sites have assumed that purgeable space includes some or all of the following:

• snapshots
• iCloud Drive files that have been evicted from local storage
• purchased Store content including music and other media, and App Store apps, that aren’t stored locally
• local caches controlled by macOS
• the local version database and other similar hidden databases
• Spotlight indexes.

Key to understanding which of these are considered to be purgeable is Apple’s clear statement that the user can’t manually remove those files, but they’re in the sole control of macOS. That eliminates the first three, snapshots, evicted iCloud files, and purchased content, which are all in the control of the user.

Snapshots have been a common misattribution here, but even a cursory consideration demonstrates that they are counted in used space, not purgeable space. The same SSD that I originally reported on now has 0 (zero) purgeable, yet it has two snapshots totalling 9 GB stored on it.

Accounting for evicted iCloud files has also been clear for some years: space used reflects what’s currently stored locally. Some users discover that the hard way, when they realise that their local storage is too small to accommodate the whole of what they have stored in iCloud. macOS doesn’t account for iCloud data in terms of what could happen, either eviction or downloading, but in terms of the current state.

How does this correlate with the measures given by Mints, in terms of space available for opportunistic and important usage?

Apple explains those differently, as the amount of space available for storing non-essential resources (opportunistic), and that for important resources. The latter matches that given as “available” by the Finder, after purgeable space has been reclaimed. According to the value obtained from macOS by Mints, at the time the Finder was claiming that 227.83 GB was available, that figure should have only been 144.12 GB. That’s a difference of 83.71 GB, exactly the same as the claimed purgeable space. Thus, the Finder’s error was in grossly overestimating purgeable space, which should have been close to 0 GB. And, since restarting, that’s what it has remained at.

If you’re unsure whether to believe the Finder or not, open Mints and click on the Volumes button: the figure given there as available for important usage should exactly match that given as available by the Finder.