The root will have a name like sda1 or hda3, with aliases like disk or
disk-1. Not very intuitive I agree! You can find the name from the command
line by typing df -h which gives you the 'Disk (space) Free (in a) human
(readable form)'.
All in lower case.

That's a lot of root you have used up. Could it be that you have stored a
bunch of data on your root directory which could perhaps be more
appropriately located in your home directory?