A clean installation of an OS involves installing it onto an empty hard drive with
no previous OS installed. An upgrade installation is an installation of an OS on
top of an earlier installed version, thus inheriting all previous hardware and
software settings.
Youmay think the choice between doing a clean installation and an upgrade
installation is simple: you do a clean installation on a brand new computer with
an empty hard drive, and you do an upgrade of a pre-existing installation. It isn’t
necessarily so! I’ll tell you why as we look at this issue more closely.