The trick with ncdu is to install it BEFORE your disk gets full from some random error log going haywire.
$ sudo apt install ncdu > Not enough free space Fuck! Actually, one trick I learned was to create a dummy file full of zeroes or rand, maybe 200MB or so, and place it in an obvious location (e.g /) for easy deleting in such a crisis.
[0] https://docs.cloudera.com/cloudera-manager/7.4.2/managing-cl...