From the screenshot it looks like OmniDiskSweeper did not finish indexing the drive. /System/
should definitely be bigger. Even if it does finish though, it won't include files in directories that can't be accessed by the user.
Try to use du
instead:
$ sudo du -h -d1 /
155M /.DocumentRevisions-V100
112K /.fseventsd
0B /.PKInstallSandboxManager
1.2G /.Spotlight-V100
0B /.Trashes
0B /.vol
36G /Applications
2.6M /bin
0B /cores
4.5K /dev
1.0K /home
9.2G /Library
1.0K /net
0B /Network
350M /opt
12G /private
980K /sbin
6.8G /System
802G /Users
7.5G /usr
12K /Volumes
875G /
-d1
sets the depth to 1 and -h
uses human-readable file sizes.
If you have enabled Time Machine, the disk space might be taken up by local snapshots. Local snapshots are enabled by default for laptops and disabled for desktop Macs. When they are enabled, Time Machine saves hourly snapshots to /Volumes/MobileBackups/
when a backup disk is not available and there is more than 10% free disk space. The local snapshots are not counted as used disk space in Finder, but they are included under backups in the About This Mac window. See http://support.apple.com/kb/ht4878 for more information.
If it's not that, you might have the same issue I had with my MacBook Air. It was missing about 20 GB of disk space that was classified as other in the About This Mac window and shown as hidden space by Daisy Disk. When I tried verifying the Macintosh HD volume in Disk Utility, there was an error about an invalid free block count. I was then able to free up the missing space by repairing the disk from the recovery partition.
Applications
and/orusr
folders? These two appear to be taking up most of the used space on the disk