You probably have too many files in /tmp
. Because the disk was full at boot, you can see that a tmpfs
was mounted in /tmp
so you can write something to that location. The du
command can't see what is there because this tmpfs
is in the way. You can get around this by bind mounting /
somewhere so you can look at it without any of the other mount points in the way:
sudo mount --bind / /mnt/foo du -xh --max-depth=1 /mnt/foo