“The overall file system structure on a UNIX system1 is always a
single tree, no matter how many or how few physical or logical
disks might be connected to it. This differs from MS-Windows, where
each disk (physical or logical) gets its own drive letter and has
its own independent directory tree root. Under UNIX (or Linux),
having more than one physical or logical disk is generally
invisible to the user, since each disk is seamlessly integrated
into the directory tree. This means there is never an absolute need
to have everything on one disk and means it is possible to have any
number of physical or logical disks and that having more than one
physical or logical disk has no effect on how the operating system
is laid out and has no effect in terms of how programs are
installed and installed programs can always find their data and
configuration files in standard places without needing to know
which logical or physical disk a given directory resides on…”