There's a lot in this that confuses me. First, a 128K cannot format an 800K DS disk (it can format a 400K SS disk in an 800K drive), so I'm curious what method you used (this is mine).
Also, the Apple II UniDisk drive did not come out until September 1985. I'm assuming you got your 128K in '85, not '84. The 400K External drives did not actually hit the streets until May 1984 and of course everyone NEEDED one, desperately. SO I am not surprised they were rare. But by late '85, I can only imagine they were rare because Apple was trying to phase them out with the 800K drives coming. BUT, an interesting bit of trivia here though ... Sony had an entire factory built around the 400K drives. When Apple switched to the 800K drive, they cancelled their outstanding order for the 400K drive, especially considering the Mac XL which also used them was cancelled. However, Sony forced them to take delivery of the full order to avoid laying off employees. This is one reason I suspect the 512Ke did not come out until 4 months after the Plus, when the 512K/800 evidently appeared simultaneously with the Plus in Europe and elsewhere – Apple was trying to unload the drives. So it would seem in late '85 Apple would have had more 400K drives than it knew what to do with.
Finally, the 1.4MB Suprdrive works fine in a 128K with the correct cable, and behaves just like the 800K drive.