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Around the end of the time of Amiga popularity, obtaining a replacement Amiga floppy drive was a nuisance, while PC drives were ubiquitous and cheap. One could install a PC drive in Amiga after applying a small, trivial hardware modification - a friend got one made as we waited, some 10 minutes of work or so.
What is the modification? How to adapt a PC FDD for use with Amiga?
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.
A PC 1.44 drive, can be converted for use in an Amiga, in fact ANY Amiga can use it, but it will be limited to 880k (720k PC). The reasoning, is because PC 1.44 drive motor spins the disk at a different rate. Amiga HD (1.76MB) spins at 150RPM, where a PC drive rotates at 300RPM.
Here (archive.org copy) is a site with specific instructions for converting PC drives from several manufacturers to Amiga replacement drives.
Note that getting the disk ready signal to work is necessary for most games that didn't use AmigaDOS for floppy I/O but that accessed the floppy controller directly. These games typically waited for the disk ready signal before continuing with floppy I/O and will thus stall if the signal isn't working.
Booting the Workbench on the other hand and running applications or games that use AmigaDOS for I/O will work fine even without working disk ready signal. Ironically, AmigaDOS (1.x at least, I haven't tested other versions) is ignoring Commodore's own recommendation to check this signal.