macOS High Sierra 10.13.2 - Missing Disk space three weeks ago I've installed macOS High Sierra on my MacBook Pro 13&quot; (Early 2015).
I've an internal SSD with about 120 GB (Apple Logic) disk space. Before the update I had about 30 GB of space lost. But after the update it seems that some of the disk space is &quot;lost&quot; or got filled with some random crap.
This is a screenshot that shows the information about my Macintosh HD (the text is in german):

As you can see the overview tells me, that there are about 28.38 GB free space with the information, that 21.36 GB can be deleted.
So my first question is what can be deleted? After some googling I found out, that these are files that were not really needed because they can be downloaded or found somewhere in the cloud. But at the moment it's not clear to me which files are really meant.
I first recognized the problem when I wanted to download the latest version of Xcode and the AppStore.app told me that there is not enough disk space and I've to clean up my disk.
I've then also checked the complete usage of my disk:

As you also can see, the app shows me that the system requires about 88.14 GB disk space, which cuts me really off. For what is this disk space needed?
I've checked my personal data and as you can see I've only a few programs installed and the needed disk space of my personal data is also really low:


The most space is taken by all the developer tools. My personal data like images, etc. were stored on a SD card.
When I run df -h in iTerm2 it tells me that in fact only 6.4 GB are left on my disk and it also shows me that there are many TimeMachine backup snapshots left on my disk. Normally TimeMachine stores the backups on an external usb hard drive:

So my question is, how can I get my disk space back and why is the system using so much space?
I had a similar problem. I ran 'dd' to fill up the SSD and then deleted the files afterwards:

dd if=/dev/zero of=/tmp/zero.1.out bs=1048576 count=10240

I created a few of those (10GB each and some larger - 30 GB) until the SSD was nearly 100% full.

I then deleted the files with: rm /tmp/zero.*.out

Finally: df -h /
now reports the correct allocation with one oddity:
ifree read 9223372036854041010 which seems incorrect.

Disk Utility (First Aid) was not picking up any issue although the reported available space was clearly off by 50 GB (almost 50% of the entire capacity of my SSD partition).

Hope this helps.