As an aside ... when you start installing multiple (big) hard drives, you probably want to seriously consider introducing a Logical Volume Manager (LVM) system, which creates the important notion of "storage pools."
Under an LVM arrangement, a "mount point," such as, say,
/usr,
no longer has a one-to-one correspondence with "a disk drive (partition)." Instead, the mount-point is associated with a "storage pool," which is represented by available space on
one or more physical drives. (Furthermore, this space-allocation can be adjusted.)
The "mount points" become a
logical view of the storage, now fully independent of the
physical reality (of the present moment), as represented by the (present...) arrangement of the storage pools. Programs see only the logical view of things: they "know nothing, and care nothing," about exactly how their needs are actually met.
(And, after all, why should they?)
It's a very liberating concept, and well-implemented. You probably should look into it.