Correct. Time Machine on HFS+ (often called “TMH”) is based on massive amounts of hard-links in order to avoid duplicating content that is unmodified from one backup to the next.
Time Machine on APFS (aka “TMA”) is based on snapshots. So the underlying file system handles the deduplication of content. In general, it is faster and more robust than TMH, but has the disadvantage that there is no way to purge a single file from a set of backups (which you could do with TMH by deleting a file from every backup).
I recently reinstalled Ventura and all my apps and data on a new Mac mini. While I am confident that there is around 300Gb of free space left I cannot get consistent figures. About This Mac, Finder, Disk Utility, and System Settings all give different results varying by as much as 250Gb. Further the breakdown by app type within System Settings produces nonsensical numbers (e.g., it tells me I have 13Gb in TV but the folder is empty).
Another example. Finder tells me Xcode is 22Gb but it also tells me that the package contents amount to 8Gb while System Settings > General > Storage > Applications says it’s 12Gb.
This is ludicrous.
I did some similar experiments on my Ventura system, and offer the following observations:
“Purgeable space” is to be considered space in use - and nobody but Apple can justify why it’s reported in the way it is. The Purgeable figure includes Time Machine local snapshots, but also seems to include other caches that macOS can purge if needed. If you subtract the Purgeable figure from the Available space in the “get info” area of your hard drive, that figure should match what you see as free in Disk Utility. In my case I have Available space as 154.89 GB and Purgeable as 6.49 GB. Subtracting the two gives me a figure of 148.2 GB which matches what Disk Utility is telling me is Free.
The “Used” value of 88.9 GB in Get Info for Macintosh HD matches what Disk Utility tells me is used for the Macintosh HD volume.
The hidden APFS volumes which include Preboot, VM (virtual memory backing store) total about 5 GB and 2.1 GB repectively on my Mac. This space comes out of Macintosh HD but like snapshots, isn’t reflected in the Used space of Macintosh HD. It’s reflected in Disk Utility when viewing the Macintosh HD volume as “Other Volumes”.
The use of sparse files and “cloud offloaded, on-demand” file services means you can’t take the Size figure at face value to determine how much disk space a file/application is using. The “Size” in a Finder window is a representation of the maximum file size (i.e. “the file address of the last byte written”). If you look in the “Get Info”, you see a “on disk” figure, which may be lower if the file is actually sparse or offloaded to cloud. (The Finder should really have a “Size on Disk” display in addition to “Size”.
The Storage panel in System Settings does appear to have inconsistencies. It for some reason over-estimates file sizes. However, if you drill down into Applications, I found that the size reported for Xcode is the “Size on Disk” figure reported in the Get Info box for Xcode, and not the Size of the app as reported in the Finder window for the Applications folder.
The bar in the bottom of a Finder window that shows available space is showing the same value as Available in the Macintosh HD Get Info box. Which includes purgeable space.
If I wanted to figure out how much space I had available on my disk, I would take the Available figure in the Get Info box and subtract out the Purgeable space. I shouldn’t have to do that - the Finder needs to do a better job of making this known.
In dumbing down the Finder experience, Apple has masked over valuable information that they shouldn’t have, IMO.
If I subtract “purgeable” from “available” space as shown in Get Info for Macintosh HD I also get the Free space shown in Disk Utility.
I suspect that Ventura classifies my audiobooks in the Books app as purgeable as it is possible to shift this to Apple Cloud. However, I have over 250Gb of audiobooks and have no intention of paying Apple for that much storage when I have already paid them for the SSD on my Mac mini.
Curiously the free space shown by Finder flips during the day between the high “Free” number shown in Get Info and a number closer to that shown in Disk Utility. However, it returns to the high number in the morning.
By the way, I asked a Genius Bar advisor about this earlier this week and they said they do not use Finder but use DaisyDisk instead. Another vote of confidence.
Media you purchased from Apple’s on-line stores (Music, Movies, Books, etc.) is purgeable because (in theory) you can always re-download it. And this content doesn’t count toward your iCloud storage, because any re-downloading comes from the store’s servers.
Your own content (not purchased from Apple), on the other hand should not be purgeable unless you are already storing it in iCloud somewhere, since you would need that storage to re-download it.
I wonder if that’s official or just what that particular tech (or his store manager) uses.
FWIW, if I want to track down what’s consuming all my space, my go-to app is Disk Inventory X, based on the open source kDirStat for Linux and its Windows port WinDirStat.
The audiobooks are from my own sources not Apple so should not be classified as purgeable. However, there’s no other combination of folders that adds up anywhere close to the purgeable number.
DaisyDisk was a personal choice of the advisor.
I’m prepared to leave this entire topic as an Apple mystery. Maybe it will be cleaned up in a future update or macOS 14 and maybe pigs will fly.
Just cleaning up some old mail and found this additional article from Howard Oakley, which basically suggests that the Finder is wildly inaccurate when it comes to free space calculations.