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