…such as a /home partition, or external media mounted in /media backed up, you either need to back them up separately, or omit this flag. If you do omit this flag, you will need to add several more –exclude= arguments to avoid filesystems you do not want. These would be /proc, /sys, /mnt, /media, /run and /dev directories in root. /proc and /sys are virtual filesystems that provide windows into variables of the running kernel, so you do not want to try and backup or restore them. /dev is a tmpfs whose contents are created and deleted dynamically by udev, so you also do not want to backup or restore it. Likewise, /run is a tmpfs that holds variables about the running system that do not need backed up.