120GB SSD-Disk: More space “used” than the sum of all files

Question

When I open the info-window about my start volume (120GB SSD), it says that 108GB are used, 12GB available. But the sum of the sizes of all folders and files (hidden objects included) in the root directory / is only around 90GB. What are these 18GB which are "used", but not locatable?

Moreover, when I open the menu Apple > About This Mac, and click Storage, it says that 60GB is used for "Other Users" (I have one further user account on my computer), but Finder says that the whole /Users directory only consumes 57GB. The same window says that "System" consumes 27GB, but the sum of all files and folder under the root directory besides /Users and /Applications (that is /Library, /System, /privater, /user, /bin etc.) is only 21GB.

So the main issue is that there are 18GB of storage space not available to me, but I cannot locate where this space is used up. I have already tried to restart the computer in safe mode, and to delete NVRAM and PRAM, but nothing changed.

Where can I find these ominous 18GB? And is 27GB for "System" realistic?

(My system: macOS 10.14.3 Mojave, MacBook Pro, SSD 120GB APFS)

Answer 1

Well, I guess I can answer my own question now...

I have two different user accounts on my computer. When I had logged in to the first account, the second account's user folder was said to have only 1GB. So I noticed that (due to insufficient permissions) the size of the user folder I was not logged in to was calculated much lower that it really is.

That means that to get the size of my /Users folder, I have to log in to the first account, look up its folder's size, then log in to the second account and look up the size of the second user folder. (and add these number)

With this new number für /Users, the sum of all objects in the root directory is identical to the "Used" space on my disk.

Answer 2

Yes 27 GB is realistic for Mojave and there may be no missing data, just delays in calculating and transient data being purged and accounted for.

I pay for a tool DaisyDisk to get me a second opinion on space, but all your numbers make sense to me.

• https://daisydiskapp.com/

Unless you can’t free space following the guidelines and see things stack in a stable or good direction, I might do two things.

• https://support.apple.com/en-us/HT206996

1. Be sure Time Machine is backing up everything to a local attached disk.
2. Boot to save mode next weekend one time and then again a month later.

See if the usage leaves out is my advice. I don’t use Clean My Mac, but you could try that for free for 7 days and also rent hundreds of utilities for a month for $10 - https://setapp.com/apps/cleanmymac

for me, the paid version of Daisy Disk and Apple’s cleanup do the trick very well for me on many Macs at home and work.