A hard drive can only have a single root directory. The Linux and windows do not treat this root directory in the same way.
Reason of single root:
For knowing the reason, think about the entire file structure as on a single tree that got turned upside down.
The root of the entire tree, irrespective of any volumes, is at the top. You have a single hard drive of computer and hundreds of drives plugged into a slew of RAID sets. This does not matter in Linux, because you always have a single root and all others grow up from that root.
Root area:
Mostly, the root area is the root area of a partition of the main hard disk. Now consider your hard disk might be something others than a hard disk.
Example:
For Example Mobile phones based on Andriod systems, which are based on Linux, have a small microchip that the Linux treats as hard disk. It does not matter what it is, nut mostly the area is the same thing as the root directory of the hard disk.
Though the hard drives only have one root directory anyway. While the Linux, the entire file structure has its root that does not change it. You might use DOS and Windows paradigm which is inherited from the ancient CP/M OS where the root directory is mapped to a drive letter, but the one disk data structure cannot be changed.
