Friday Magic: How to make disk space unpurgeable

It must be almost two years since I last demonstrated some magic tricks involving available and purgeable disk space. At that time, the amount of space involved was a mere 83.71 GB. Today I’m going to show you how I converted 228.16 GB of purgeable space into used space, recovering a lot of my files in the process.

Prior to my iMac Pro’s forced update to Sequoia 15.3.1, described here yesterday, its internal SSD had around 150-160 GB free, with no purgeable space at all. Immediately before installing that update, SoftwareUpdate reported that there was 160.57 GB available. When I had coaxed it back into life, now running 15.3.1, the foot of each Finder window told me there was now “393.72 GB available”. Imagine my surprise/shock/horror that about 240 GB of what had been on that SSD before it was updated had now vanished.

Recalling my previous experience, I selected Macintosh HD in the Finder, and opened the Get Info dialog. That confirmed the situation, stating

Available 393.72 GB (228.16 GB purgeable)
Used: 928,672,419,328 bytes (829.67 GB on disk)

A little arithmetic reveals that of the 393.72 GB “available”, only 165.56 GB was actually free at the time, the rest being “purgeable”. Together the truly free and that used “on disk” amounted to 995.23 GB. Adding the 16.16 GB used by other volumes, my Mac’s internal SSD had grown in capacity to 1.011 TB, which made that slightly traumatic update worthwhile after all.

Sadly, Disk Utility wasn’t so impressed. The figures it gave were very different indeed:

Available: 165.56 GB (none purgeable)
Used: 818.52 GB + 16.16 GB on other volumes = 834.68 GB
One snapshot of 7.16 GB

for a total disk size of exactly 1 TB. The figures my own Mints gave were in accord with those from Disk Utility.

Although I much preferred the Finder’s figure of nearly 400 GB of “available” space, I realised that could only come at the cost of purging all that 228 GB of “purgeable” space. As that seemed to include many of my files, I thought it was time to work this week’s magic trick. I therefore restarted the Mac, and all of a sudden purgeable space had vanished, leaving me with only about 165 GB of free space after all.

To remind you of what I found nearly two years ago, after updating to macOS 13.3.1, the Finder found 83.71 GB “purgeable”, and my SSD had then grown to 1.08 TB in size.

That’s two major versions of macOS and almost two years apart, and the Finder still can’t come up with correct figures.