There are two hard disk interfaces that were used in the early days of the PC, in the
1980s. These are still found on any systems of this "bygone era" that are still
in operation; this may surprise you, but there are in fact many thousands of 15-year-old
hard disks still chugging along! 
However, they are no longer used in modern systems,
nor are new drives made that use these interfaces.
What makes an interface "obsolete"? Essentially, when something new comes
along that is sufficiently superior that it pushes the old standard out of the market
completely. Just being "newer" or "better" isn't really enough: it has
to be so superior that the market sees no reason to keep using the older standard. Both of
the interfaces described here were made obsolete by IDE/ATA and SCSI, which offered
significant advantages over them without imposing any real cost. (It's actually fairly
uncommon in the PC world for a standard to be made obsolete by another, because in most
cases the older standard still has adherents due to some particular advantage it offers.)
Since these interfaces are of little or no interest to 99% of current PC users, their
descriptions will be brief.
Next: ST-506 / ST-412 Interface
