Server disk full after upgrade from Ubuntu 14.10 to 15.10
Solution 1:
Run the following command to find free disk space information:
df -h
Then run the following command to find disk utilization information:
sudo du -shc /
(note the sudo, this should resolve the permission issues you were seeing)
When you find a directory taking up a considerable amount of disk space, cd to the directory and repeat step #2. Keep repeating this step until you identify the source of the disk space issue.