Back in February 2008 I build my own external disk drive for my Amiga 500.
The circuit takes care of the differences between the Amiga and the PC disk drives, the only modification (not required*) to the drive being the bypass of the density sensor.
Originally discussed in a
thread at retrocomputacion.com forum, today I finished writing a more concise and complete article detailing the workings and construction of the controller board.
You can read the article
herein spanish only, if you're interested on an English version, please leave a comment here and I'll add the translation here.
* Only required to use HD floppies without the need to close the HD density hole with tape.