I picked up an unused "eICo High Density Module" in amongst other Atari
stuff. It's a small circuit board that allows you to replace the DD
floppy in an ST with an HD floppy drive ***as long as you have a
suitable floppy controller***. It comes with detailed instructions,
software on a diskette, and the required wires & connectors, but NO
floppy controller. You do have to have some expertise with a soldering
iron to install it. And, as noted above, you DO need a suitable floppy
controller; the standard ST one won't take being clocked at 16MHz.
Please email me if you would like it for the cost of shipping (around $2
to North America, $3.50 elsewhere). Otherwise it will probably find its
way to the dumpster ...