No free space when there is free space - purgeable files

Introduction: Recently I ran into a difficulty with free space. It turns out that Time Machine stores local snapshots of the used part of the storage drive, I assume until a regular backup is done. But these snapshots fill up space without being listed as filling up space.

In my case, I was running a game, Skyrim through Steam through the Porting Kit. Steam run this way produces a ton of "reports" 17.8 MB in size several times a minute inside the app directory (which is hidden, it is .../Applications/Games.localized/The Elder Scrolls V Skyrim SE.app/Contents/Resources/drive_c/Program Files (x86)/Steam/dumps/reports), which gradually fills up the disk. Time Machine was dutifully producing snapshots including all these garbage reports.

After deleting them, I still had problems with applications claiming there wasn't enough disk space.

Solution: Okay, so if you find that you have a bunch of "free" space but actually a lot of that space consists of "purgeable space" not really free space, list the Time Machine local snapshots using Terminal:

>> tmutil listlocalsnapshotdates

You will get dates and times. You can delete the local snapshots on Terminal with the command:

>> tmutil deletelocalsnapshots <date>

where <date> is one of the results of the previous command in the form YYYY-MM-DD-TTTTTT

Note: An earlier thread on this subject started by mike.adams with the subject line "How do I access/purge purgable memory?" has an "Apple Recommended" reply by rkaufmann87 that is not actually helpful, to wit: "Drag the files to the Trash and empty the Trash." No, it doesn't work that way.