The differences between PC floppy drives and Amiga floppy drives are as follows:
- PC floppy drives normally answer to drive select 1 (DS1), internal Amiga drives answer to DS0
- pin 34 on the connector is used for disk change on the PC, disk ready on the Amiga
- pin 2 on the connector is used for high-density detection on the PC, disk change on the Amiga
So modifying a PC drive so it works inside an Amiga involves :
- switching the drive to DS0 (on some drives this is a jumper, but on most drives you'll need to desolder the DS1 pads and bridge the DS0 pads)
- re-routing pins 2 and 34
- grounding the HD detection signal
You'll need a PC drive which exposes the disk ready signal somewhere on its PCB.
There are many examples of this on Amiga forums; here's one, here's another. Ian Stedman has an add-on board which can be used instead of modifying the drives; it synthesises the disk ready signal using a couple of other signals.