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I am trying to figure out how to free up space on a 2TB SSD that only has about 1.1TB of legitimate usage, but says its completely full. The disk has two APSF volumes on it, one taking up 1.8Tb (per Disk Utility) and the other taking 194GB. When I total up everything in the larger volume, it appears that it should only be taking up about 1.1TB.

The smoking gun that I see is this info coming from Daisy Disk:

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I recognize the space taken by the two snapshots. These numbers match what I see in Carbon Copy Cloner, the tool that created those snapshots. Why does the total space used by snapshots not equal 46.1GB + 1.5GB = 47.6GB? What is the remaining 543.2GB of space that Daisy reports in "snapshots..." all about? Is there a way to recover that disk space?

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  • In anticipation a question I expect to get...the missing space is not Purgeable Space. DaisyDisk shows only 138Mb of "free+purgeable" space. In order to confirm that this space won't be reclaimed by writes to the volume, I attempted to create a large Disk Image. I can't create an image of even 500Mb in size, so little or none of the used space is purgeable.
    – CryptoFool
    Commented Nov 12 at 20:50

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DaisyDisk's developer here. The APFS snapshots are a little tricky when it comes to evaluating their sizes, due to their incremental nature and vast sharing of disk blocks between snapshots. When you evaluate size of a single snapshot in isolation from other snapshots, you get the maximal size, as if all its blocks were owned by this snapshot. However, when you evaluate the size of this snapshot relative to an existing older snapshot, you get a much smaller size, because probably there haven't been much change on your disk between the time moments of the older and newer snapshots, consequently they share a lot of common blocks, and those common blocks are assumed to be owned by the older snapshot (counted toward the older snapshot's size). A similar logic happens for calculating a total size of a bunch of snapshots - their total size is not a simple sum of the sizes of all snapshots in isolation.

At any rate, snapshots are opaque file system objects and third-party apps cannot directly assess their sizes, these sizes you observe come from the macOS as ready-made numbers.

What you are seeing inside the "snapshots" in DaisyDisk are snapshots created by Bombich Carbon Copy Cloner backup app. For macOS' security reasons, any third-party snapshot can only be deleted by the app that created it. So in order to delete them, please open CCC and delete the snapshots from the app's own UI. After that, you should see a change of the overall free and hidden space in DaisyDisk too.

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