What would you think if you looked at free space on your Mac’s internal SSD and noticed it had risen by more than 50%, from about 144 GB to 227? Wouldn’t you wonder what had been lost, and start checking your media libraries and Documents folder?

My iMac Pro started off with well over 250 GB free on its 1 TB internal SSD, but over the years that has become whittled down to a steady 140-150 GB, where it was the last time I had looked, after updating to 13.3.1 . Although I keep most of my active documents and some media libraries on an external SSD, there are still plenty of important files on its internal storage. Nothing I had done intentionally could suddenly have freed up more than 80 GB, so this was worrying, more so when I looked at the figures in Get Info.

The Finder was certain there was nearly 228 GB available, but also claimed that 852 GB was used. Did that mean that my 1 TB SSD had grown in size overnight to just over 1.08 TB? What was this “83.71 GB purgeable”, and had the Finder already assumed it had been deleted? Could the Finder really have made an error of more than 50% in calculating free space?

I turned to Disk Utility, which confirmed what I had expected, that there was actually 144 GB free.

My own free utility Mints said much the same.

As I know the origin of those last figures, I concluded that the Finder was just making its numbers up, perhaps after conferring with ChatGPT. I restarted my Mac, and checked again in the Finder.

That seemed closer to the truth, so I called off the search for the missing 80 GB of files.

APFS was introduced to macOS in 2017, and I celebrated its birthday a fortnight ago. Over those years, the Finder has repeatedly shown errors in accounting for used and free space on APFS, although they’re seldom as large as this.

Several of the valuable new features in APFS, like snapshots, sparse files and clones, make it difficult to arrive at a precise estimate of free space. However you do that, the figure is dynamic and has to make some assumptions. Although I don’t know how the Finder or Disk Utility make their estimates, Mints uses a proven part of the API through URLResourceValues that’s readily accessible and correlates well with Disk Utility.

Knowing all the difficulties involved in obtaining a reliable estimate of free space, the Finder then fails to perform any sanity checks, either against figures obtained from URLResourceValues, or even checking its arithmetic. How on earth could a 1 TB disk contain 852 GB of storage in use and still have room for over 227 GB of free space? Even using integer arithmetic the problem is obvious.

The Finder is the front window of macOS. When it has made a fool of itself like this, repeatedly, ever since the introduction of APFS, it only makes the whole system look amateur.