Disk unknown usage

Question

I use df -h find 17G used. However I use ncdu / to find which directory holding disk usage, show me only 5.5G used. What's wrong with this?

df -h screenshot:

ncdu / screenshot:

[Edit 2016.09.05]

It is resolved. I should use 'sudo ncdu /', then I found the usage is match for df -h usage.

Answer 1

This should work

In a terminal type:

 sudo du -BM -s /*

This will give a summary of disk usage in each directory and file in the root directory, in megabytes.

You need sudo because you want to read all the files.

There is also a man page for du.