trouble understanding disk space

To begin with, I believe that the Disk Utility usage includes storage for Time Machine local snapshots (see About Time Machine local snapshots) while About this Mac>Storage marks such storage as available.

As for bootable backups, I suspect that Time Machine no longer backs up macOS per se. Instead full recovery relies on restoring the OS following by using the Migration Assistant to restore user files, non-OS apps, system settings, etc. (which, of course, are included in the Time Machine backup). I, myself, don't use Carbon Copy or SuperDuper although I've seen several posts recommending these programs (with certain caveats).

- Pie Lover

Well, it's a bit complicated. I suggest having a look at About this Mac>Storage>Manage... It offers a good summary (see below) as well as suggestions to reduce storage.

Different estimates are calculated in different ways. For example, my Time Machine volume shows 2.7 TB even though the quota for that volume is 160GB (multiples backup copies do not use additional storage beyond that of the original file).

- Pie Lover

I can tell you how to make all this much more simple. Just ignore everything except for Disk Utility. Disk Utility will show you the correct size of your hard drive and the amount of free storage. Anything else the system tells you in any of the other places is bogus.

I can't speak to the bootable backups only to suggest that they probably aren't actually bootable. Recent versions of macOS can break booting from external drives.

You are welcome to try to explain how it works in all of the similar questions that people keep posting. When I see values that confuse people, mislead people, and are sometimes obviously wrong, I just say the data is bogus.

Dannymac22 wrote:

The purpose of my question was to ASK how it all works. When I see values that confuse or mislead people, my first assumption is that there is something that I just don't understand. The question still stands.

My answer still stands too.

I can explain how it is supposed to work. Buckle up, buttercup!

I'll break things down into three categories - free storage, snapshots, and categories.

Free storage:

Modern macOS uses the APFS file system. This file system is supposed to be much better at conserving disk space by only writing new data when the old data actually changes. You can duplicate a single file tens of thousands of times with no meaningful loss of disk space.

However, what this means is that no individual file size has any meaning any more. A file listed as 100 MB in size could be 100 MB, or 50 MB, or 20 bytes. Obviously, this problem gets worse when you look at entire folders. The "size" of a folder is now completely meaningless. All of the files in a 10 GB folder could also reside in a different 10 GB folder. But the total size used by both folders is only 10 GB of storage.

If you look around, you'll notice that the operating system never even talks about "free" storage anymore. Instead, it is called "available" storage. People think they mean the same thing, but they are wrong. "Free" storage means that the storage isn't being used. "Available" storage means that the storage actually is being used, but could be deleted if you really need it. How do you decide if you really need it? Trick question - you don't. The operating system decides that, on its own schedule. If you run out of disk space, you will likely get lots of warning about running out of application memory (long story) and storage. So you go off and delete a bunch of files. Maybe you delete 10 GB of files. And yet, your free (or available) storage doesn't change. The problem is that those 10 GB are still being used by other files. In most cases, after a frustrating delay, the operating system will go ahead and delete some of the files it is hanging on to and give you more usable storage. But this always happens a significant time after all of the scary warnings and frantic, but useless, deletions.

The only place to see the actual "free" storage is in Disk Utility and some command-line tools that nobody knows about.

Snapshots:

One of the primary reasons for all this problem is local snapshots. The operating system automatically makes copies of its filesystem on a regular basis. When you delete files, all you are doing it setting a flag to tell the operating system that the file is unwanted. You actually can't delete files at all, by any means. Only the operating system has the right to delete your files. It will not delete any files that still have a pointer to them. So if you have copies of a file and delete one or more copies, nothing gets changed because there is still one, original pointer to the file. APFS local snapshots maintain pointers to ALL files. To delete a file, you may need to delete snapshots, which can only be done via some utility or on the command line.

Categories:

This is probably one of the most frustrating of all problems. When people see categories with names like "System", "Applications", "Documents", they naturally think that these refer to specific directories on the hard drive with the same names. They don't. They have nothing to do with each other.

These categories all come from the Spotlight disk indexing system. Many of these categories are pretty straightforwards and sometimes very specific - Applications, Mail, Message, Photos. No doubt about those. But what about "Documents" or "System data"? In most cases, the "System data" is just a catch-all for any files that couldn't be otherwise categorized.

If you have external hard drives, or mechanical hard drives, then spotlight gets really, really slow. This will cause the "System data" size to balloon because the operating system gives up waiting for Spotlight.

In essence, the data is accurate, unless it isn't. If you can think of a better description than "bogus", I'd love to hear it.

FYI, local snapshots can be manually deleted as mentioned below.

- Pie Lover