Ubuntu root drive running out of space, I can't find source through du or lsof

Solution 1:

Well, it's only a guess but it may works: I think that user once forget to mount /dev/sda1 as /media/DATA and all data was written on /dev/sdb2 instead of /dev/sda1.

To check this, please, unmount /media/DATA and check files and folders under this path.

Solution 2:

I regularly use 'ncdu' for that, it's small enough to still get it installed.

sudo apt-get install ncdu

Just make sure you run it as root or via sudo:

sudo ncdu /

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Solution 3:

When you want to know where space is being used in a particular filesystem, you can use this command to find the 20 largest directories, which can help you locate where the most space is used,

du -m / |sort -n |tail -20

But the root filesystem is more difficult, because all filesystems are mounted to root. But the -x (--one-file-system) argument will report only the desired filesystem,

du -m -x / |sort -n |tail -20