@rkauer
The drive that is borked is: Mitsumi D359T5
It doesn´t detect disk changes
Now, you reccomended me to clean the disk present switch. After having looked inside the drive, I see two mechanical switches. One on the left hand side, detecting read/write protected, and one identical on the right hand side. This is the one I suspect is "disk present".
Bot switches are the same mechanical kind which out find in mouse buttons and joysticks, with the slight difference that these are "smoother and moore springy" with taller plastic pins on top of them.
Now I tested two other drives in the a4k desktop while I was at it: Two Chinon FZ-354 working flawlessly and one with the same problems as the Mitsumi.
Thankfully I had an a1200 floppy spare from AmigaKit (in addition to the two Chinon) which I put in there.
Does anyone know if the a4k floppy from AmigaKit is DD or HD density, the Mitsumi I just lost was HD - not that it´s very important - but it´s nice to know