The root disk is in your case /dev/sda6 (the one marked as '/' which means root). To make a copy you can use the 'dd' tool, but please read the man page before doing something that you regret.
It is not clear what is truely asked of you:
- making a backup of the root partition only
- making a backup of the whole root file system (this would require to backup all the mounted partitions as well, like /usr, /var and the others)
- setting up a hard disk mirror with some other hard disks to continually mirror all data changes