I have an ubuntu server which yesterday i performed an rsync command for the first time to backup a drive sdb1 to sdc1. Neither of these drives are the OS drive thats full.
This morning i woke up and the OS drive is full. I can't even boot into the GUI but i can still ssh into the machine.
edit: I found the source - the rSync command seemed to be creating a directory /media/sdc1 physically on the OS drive instead of using the mounted /media/sdc1.
Does this command look right to you:
rsync -av --delete /media/sdb1 /media/sdc1
Drive 1 is mounted sdb1 and drive 2 is sdc1