So just for the record. This what (apparently) happened.
1 of the drives was damaged (the heads I believe, nothing I could do to repair the damage). It --sometimes-- damages the disk inside, visibly scratching it.
The second drive I have is a new internal drive bought on Amigakit. I thought it was working and was sure the culprit was my amiga, but tested on 3 systems it fails on all three. It simply gives NDOS or FD0:????. So my suggestion would be to avoid them (YMMV).
As soon as I recapped my miggy (don't do it yourself... it's a pain in arse!) the last drive I have finally worked. Even the sound it makes is different now... it's healthier... if you know what I mean.
I tried to recap the bad drive too, but that didn't help at all. Now I have to decide if I want to recap the working floppy drive I have or not.
Thanks everybody for your help. This is surely a painful hobby.