how do i know if a root folder of a external hdd is which ?

in windows system, an hdd so called "parent directory" is perhaps d:\ or e:\ . when i saw that d:\ or e:\ , i know this is the so called "root drive" of the hdd. everything is located in it.

however in linux environment, how do i know if "/6TB WD hdd" is a "root drive" , may it is just a nested folder that have being mounted to "/6TB WD hdd"; maybe this "/6TBWD hdd" has a "parent folder"..

i can mount any sub-folder to a mounting point. and if there is no way for me to tell if that's the highest folder (which contains all other folder in the physical hdd)?