I read online that the KN3000 SY-HD1 is no longer available...but it is possible to use an IBM DSOA-21080 HDD IDE 2.5 Hard Drive in it's place. Does anyone know anything about this???? Thanks

my recall is that there was a slot on the rear right end of the keyboard with a flapper door that the hard drive went into and you pushed it down to engage the connections and then the flapper door closed. It would have to be the right size, configuration etc to slide down that chute and plug in.
Bebop Bill

The SY-HD1 harddrive was a "plug-in unit" and formatted in a special way, so even if you manage to squeese in a replacement, I doubt it would work, the KN's operating system will not recognize it.
Also if I don't remember it wrong, we had to swap 2 chips inside the KN3 as well, as a kind of bugfix or OS upgrade. Maybe that was only a part og make it read/write both midi-file types 1 and 0, but I think it also was a part of the HD set.