UPDATE
Well, today I pulled an old A500 Revision 5 out of the garage loft and tried this PowerXL High Density floppy drive in it ------ and wouldn't you know it? THE BLOODY THING WORKS! Recognized a high-density floppy disk right away! I didn't even need to use the PowerXL patch.
So it WAS the revision 4.4 A2000 motherboard - because heaven knows I tried every single jumper setting and cable permutation I could think of and nothing ever worked.
So ---- after ten years ----- mystery SOLVED.
As an aside, the old Chinon 880K floppy drive in this A500 was also very weird. It absolutely HATED any high density media formatted to Double Density (even when the extra hole was covered with tape). 9 out of 10 high density floppies formatted to Double Density (and known to work fine on other Amigas) would either not be recognized at all by that Chinon drive, or load partially then refuse to go any further. I don't know how it knew - there was no sensor pin for the high-density hole in the drive. It just must have not like the magnetic bias of the particles or something. Second weirdest thing I've ever seen pertaining to Amiga floppy disc drives!
I've read over the years that DD floppies are higher quality than HD floppies, but with the drives it's the other way round.
Hence, an HD drive will read and write HD and DD no problem, but a DD drive will read and write DD, but may struggle with HD.
From this I understood it was always better to buy DD floppies and they will last longer even in and HD drive.
Of course this coudl be total nonsense, but it is what I read and seems to make sense that as drive technology improved, the media didn't need to be made to such a high standard (expense).
