Disk Space taken up by misc/other - not TimeMachine
Are there multiple users set up on this machine? The screenshot of About this Mac -&gt; Storage shows that there are lots of &quot;Other&quot; files on this partition, which usually means files outside of your user's home directory (and probably hidden/inaccessible).
By default, OmniDiskSweeper can only size folders which you can read. You may use Terminal.app to launch OmniDiskSweeper with root permissions to track down files your regular user can't access:
sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper

Also, /usr and /private are 10x larger than usual.
As for the second Data partition: That is weird. I would consider reformatting (after a full backup, of course).