Running du can be slowed down by huge filesystems mounted, and the outputs can easily be distorted by such directories.

To exclude them from du result,

1. Add -x flag, e.g.

   du -hx --max-depth=1
   

2. Use --exclude=<PATTERN_TO_EXCLUDE> to specify patterns of directory names to exclude, e.g.

   du -h --exclude=/mnt
   

References:

• https://askubuntu.com/questions/170636/analyse-disk-usage-ignoring-mounts