Whole disk marked as purgeable incorrectly

In Disk Utility, by default, if you click the View menu and Show All Devices you will see this on the sidebar:

The top outline entry is the physical Apple SSD. Next you have an APFS Container. Inside the APFS container the volumes share the free space. Macintosh HD volumes is a collection of a read only Macintosh HD (grey) and that is snapshot to APFS, Macintosh HD snapshot. This means that the operating system files are immutable unchangeable. Only Apple can modify those system files. APFS then gives the user a Data volume. Here is where all your data and 3rd party applications reside.

This is why you can Erase all Settings and Content and it does it very quickly. The Data volume encryption keys are discarded making the data unreadable and macOS will then just reset it to factory settings and the Mac is just like you took it out of the box. Works the same way on iPhones & iPads.

You can also create your own APFS volume and install a different version of macOS and you can then dual boot. However, this requires more disk space as it doubles the storage requirements for two operating systems.

APFS uses snapshots along side Time Machine. If you ever activated Time Machine and attached an external disk and ran a backup. Then disconnected the drive quite some time ago. Apple macOS will continue to snapshot changes to disk anticipating the next Time Machine backup. These snapshots are written to the internal APFS file system. When the backup disk is detected and a backup begins then the snapshots are copied over. A snapshot doesn't take any appreciable storage space until you start changing the original files that were snapshot. At that point the differences begin to pile up in the snapshots. Thus needing to backup more and more data. Once you attach the external drive, it will eventually complete the backup and most of the snapshots will clear out.

In Disk Utility, click on the View menu and ensure that Show APFS Snapshots is enabled. Highlight the Data volume and you'll see if there are snapshots listed below. As others have indicated the values being provided regarding free space can be very confusing. APFS makes this rather complicated due to the immutable nature of the file system and how snapshots function.

If you go to System Settings > General > Storage there are more options to help you free up space. Click the little icon to the far right ⓘ and you now have options to free up some space. For example, you can see the attachments in Messages and delete them or show in Finder, etc. That is a major improvement over the past behavior.

A couple of other utilities to help find files that are taking up a tremendous amount of disk space. You'll need to grant these tools Full Disk Access in Settings > Privacy & Security so they can see everything. You may be surprised when you run scans with one of these tools. It will show larger files in a larger graph layout. It helps you visually identify files based on size. The first one is free but hasn't been updated in a very long time. The next two are on the App Store but can be procured outside the App Store and GrandPerspective is free if you can find it outside the App Store. If an App exists in the App Store I typically recommend that one over others.

OMNI Group's OMNIDiskSweeper

https://www.omnigroup.com/download/latest/OmniDiskSweeper

GrandPerspective

https://apps.apple.com/us/app/grandperspective/id1111570163

DaisyDisk

https://apps.apple.com/us/app/daisydisk/id411643860?mt=12

If you wish to understand what's going on inside the APFS file system. Here is a blog showing all the APFS related articles that Howard Oakley has written.

https://eclecticlight.co/?s=APFS

Next time you buy a Mac, you need to consider the amount of internal SSD storage you require. You can save considerable funds by using cloud storage, network storage, or external storage. You'll want to consider a backup strategy. While Time Machine is perhaps the simplest no brainer backup solution ever devised. If you desire more control over your backups, consider CCC - Carbon Copy Cloner or SuperDuper.

The way I think about internal storage is the data I need right now. When I am done with working on a project, I'll archive everything off the Mac. I can always retrieve it later but if that data on the internal disk is doing nothing but sitting there. That is wasted space in my opinion. Most corporate business laptops deployed in the millions are shipping with 256GB or 512GB. User data is kept on the network or sync'd with a Cloud service. Most users have only 50-60GB of active data files. Once we eliminated email archives such as PST files users weren't maxing out their internal storage. Don't be a pack rat, discard what you don't need and keep what you do need and ensure it's backed up.

jab11sk wrote:

After some restart, the system marked 85% of my disk as purgeable. As you say, it can be "released at some unknown time in the future." I have hundreds of gigabytes of real data on this disk, is the system going to randomly purge it "at some unknown time in the future" ? That's what I'm trying to prevent!

It would not be your data that is purged, it would be data macOS is using & controlling. The exception to actual user data would be for iCloud where it may remove the local copies from the system if more space is needed, but those relocated iCloud synced items would be retained in the cloud (unless of course you delete them from another device).

Plus, you may want to check your CCC backup settings since you may have multiple CCC backup APFS snapshots residing on the drive (again considered purgeable by macOS). Even if you delete data to make more room, that deleted data may still reside on those backup APFS snapshots until those snapshots are deleted at some point in the future.

Storage on APFS volumes is complicated. Between APFS snapshots, and cloning of data when you copy an item to another location on the same APFS volume. And like @etresoft mentions, most of macOS is reporting incorrect values for most of the storage items. The Free space value is what you want to focus on since even the "used" space value can be inaccurate due to how macOS calculates storage on an APFS volume.

Apple has made a complete mess out of the storage system reports by showing the "Available" value everywhere within macOS. Completely ignore the "Available" storage value since it is very misleading. The "Purgeable" storage value is related to the "Available" value. The two values can be used to calculate the actual Free storage space value which is the critical value you want to reference.

According to your screenshot of Disk Utility, you only have 41.98GB of Free storage space. This is the critical value you want to monitor & reference. Free storage space indicates the actual storage space that can be used immediately. You never want to run low on Free storage space, or bad things will happen (I hope you have good backups).

The "Purgeable" value is the amount of storage that will be released at some unknown time in the future.

The "Available" value is storage that is a mix of actual Free space and of storage that macOS plans to release in the future (aka "Purgeable"). The user cannot control the "Available" or "Purgeable" storage....that is all under the control of macOS.

All the user can do is make sure the system has sufficient Free storage space at all times for the normal operation of macOS and your most intense workloads. macOS requires at least 20GB+ of Free space at all times just for basic simple use of macOS, so some other workloads will require you to have much more Free space....perhaps several hundreds GBs for workloads such as video editing, etc. Even 20GB disappears very quickly even when just using a web browser to browse the Internet. Like I mentioned previously, bad things will happen if you completely run out of Free space (includes corrupted data, inability to delete files, system won't function due to the previous two items, etc.).

Free space = Available space - Purgeable space

Available space = Free space + Purgeable space

Follow the previous advise - First

FYI, what is presented in your image is the User Account Volume

Non the less.

On a 500 GB Drive, adding 439.13 GB User Volume + 18.85GB Other Volume + 41.98 GB gives 499.96 GB

To reiterate some of what has already been mentioned

It is generally a good computer practice to alway keep at least 20% to 25% of the Total Drive Capacity’s as Empty Space.

Allowing the computer to drop below these guidelines may eventually, cause unintended consequences.

There is Purgeable Space and there is Empty Space.

Purgeable Space which is Controlled by the Operating System.

When the Operating Systems decides the computer needs additional Empty Space, it will move a portion of the Purgeable to Empty space

AFAIK - there is no User Actions to hasten this transition from Purgeable to Empty Space

It can day or longer before this will occur.

jab11sk wrote:

It's almost full, there should be about 40GB free, but Finder now reports 470GB free space.

Finder is lying.

Used space is still 439GB which is correct, but how do I get the available space show correctly ?

That's impossible because available space is a made-up fantasy term. It doesn't exist except in some Apple engineer's peyote-fuelled weekend vision quest sometime in 2016 or 2017.

I don't want to update to Sequoia if it could break my filesystem.

You definitely don't want to attempt to update to Sequoia.

Does this mean the system might decide to purge the 430GB and erase my disk ? How do I revert this ?

Can't say for sure. Your screenshots don't make any sense, but then you knew that.

My best guess is that somewhere you have 430 GB of cloned files. That would confuse the system and could be what is creating this mess.

Unfortunately, you don't have many, or any, options really. You need to attach an external drive and save your data as best you can. I don't recommend Time Machine in this case. Then, erase the hard drive and reinstall the operating system.

When you copy your files to the external, that will "fix" the available space, cloned files, or whatever is causing this. You may be left with 1 TB of data, with identical copies instead of clones. You'll have to manually identify which files are the ones you want to keep.

Good luck!

jab11sk wrote:

Yeah I definitely don't have 430GB of cloned files on a 499GB drive :) apfs must be imagining things ?

The amount of cloned files you could have is almost infinite.

I used carbon copy cloner to make multiple images of my drive in case I need to recover anything, I guess a clean install wouldn't hurt anyway.

I was going to suggest dividing 430 by 499 to get the probability that Carbon Copy Cloner is causing this. But the result is only 86%, which is far too low.

Wow 👍

Impressive work there

One last and final suggestion

View APFS snapshots in Disk Utility on Mac

After that, good luck on several fronts including the computer issue

Do you have 'optimize storage' turned on?