It is true that the magnetic coating on the original DD disks from
the mid-1980s limited them to a capacity of 720 Kb.  Towards the end
of the '80s advances in technology produced an oxide coating which
was good enough for 1.44 Mb HD disks.

However, since the introduction of the HD disk, there is usually no
physical difference in the magnetic medium (the thin brown bit inside
the plastic case) between DD and HD disks.  They are produced on the
same production line.  Only after quality control are they separated
into those which are good enough for HD purposes, those which have
some issues but can still be used as a DD disks, and those which are
rejected.  So a recent (well, since the late '80s) DD disk is just
a slightly dodgy HD disk in a case without the extra hole.

Adrian Thomas