Sorry for the late response! For disk usage problem, I was able to reproduce it and found out the Disk Usage visualization is displaying Top 10 system.filesystem.used.pct and Order by Doc Count instead of Order by Average of system.filesystem.used.pct. Also group by is using system.filesystem.mount_point instead of system.filesystem.device_name.
Also if you toggle the different options around, some data shows up 
Do you want to create an issue for this in github for this? Or I can do it tomorrow.