I think that him knows that there are really 2 physical drives.
I guess one disk is root filesystem (/root) then the other the filesystem itsef (/).
I guess that him has watch at TOTAL space of the system, maybe from the file manager. (wich usually shows at bottom the total free space, relative to this path) so / will be 40Gb then /root will be 10Gb or 30Gb.
It's hard to some people to understand the mount points, including non experienced Linux people.
Please run
df -h
then paste output here.
Best of luck with photostop