HThis means that (I'm pretty sure - I've seen TOS 2.6 or something ROM noted as a limitation, though) the machine can at least in theory read HD disks. I don't know precisely what that means. However, there's a floppy to Smartmedia adapter that (as I understand it) allowed the use of HD floppy drives as scalable smartmedia readers - and could be used on any HD drive at least in theory. I'm seriously inclined to try that on the ST if I can actually use HD disks.
Only the very late "Mega" STs have a controller capable of using high-density floppies "out of the box".
There are mods out there to overclock the controller in earlier models to the requisite data rate but they come with gotchyas.
In any case, those Smartmedia readers for floppies don't work without a driver. To make a long story short, you know these things?
Those SmartMedia cards use the read-write head in a floppy drive the same way, *BUT*
they don't actually emulate a floppy disk. The driver is necessary to make the host system understand that it's just using the floppy controller as a data channel over which high-level commands are sent to communicate with the storage device.
So...
I'm also then inclined to try it on some Macs, and then - to cycle back - I think I've read about HD floppies on IIgs. I'm assuming it was a single card that's obscenely rare and prohibitively expensive?
It also won't work in Macs, not unless you have a driver for it.
And, high density floppy drives for the IIgs do indeed require a special controller. Apple made one, which is indeed rare and expensive and has to be paired with a Macintosh "SuperDrive" external floppy, and there were also third-party ones that worked with normal Shugart drives. (Those might be cheaper, if you can find one, maybe?)