What do you use to transfer data for and from the drives, ISA card or a mainboard connector? Did you remove any of the caps from the drives to measure if the indeed are OK?

I have tried motherboard's own floppy connector aswell as stuff on IO cards.
Of course I removed the caps to measure them, in circuit measurements do not work in most cases for them.

Generally the "Not Ready" error in DOS displays when there is no disk in the drive. Have you checked the IR sensor / microswitch that detects if the floppy is in the drive? Also check the resistor terminator pak directly next to the connector. Is it intact?

On another note, I had the exact same problem when building my 286 machine this week. Whenever it tried to read a floppy, it gave the "Not ready" error regardless of if there was a disk in there or not. It turns out it had something to do with the serial/parallel/gameport card that I had in there. As soon as I removed it, the computer happily booted as if nothing had happened.

These pictures are from my own TEAC FD-55GFR. The first one is the IR LED for the rotation speed/disk presence detector, and the second is of the resistor terminator pak. The IR detector is underneath the flywheel.

One of my drives is the same model, jumpers are set like so aswell. I did clean the sensors earlier, all of them had a layer of dust on them and some had a ball of fluff covering them. I looked at it again I did find one more sensor under the main board which is responsible for floppy insertion detection and cleaned it but unfortunately that didn't change anything. Termination resistors are present. Other drive has completely clean sensors and all the ones I looked at reacted properly aswell.
I don't think there's anything interfering on the motherboard(s). I will try some PII/PIII board later, they shouldn't show any issues as far as electronics go.

I'll look at some signals with an oscilloscope later aswell.

As far as the cable positioning goes, IDK if you know this, but on all IDC connectors that I've seen, there's a little '1' marked on one corner, or a little white triange on the outline of the pin header on motherboard. The red stripe on the cable goes on that side. Have you tried using your BIOS to perform a read/write test on the floppy drive? I know some motherboards have built-in diag utilities.

Cabling is all correct or there wouldn't be anything going on, and certainly not when I try to access the drive.
None of mine have any floppy related diagnostics. But there should be standalone floppy diagnostic programs. I recall seeing one that is useful for aligning the heads again, I'm sure it has some extra stuff in it too. Now to figure out what it is.

I have a complete PC AT class advanced diagnostics set, I can send you an image of the diagnostics diskette if you like.

That would be most helpful, thänk you ~

hmmm, the floppy seems to have a bad sector. It won't image properly. Maybe try running "msd" in your dos folder and see if it has floppy test programs? I don't remember if it includes them.

MSD has no such things in it. What is the name of that test suite ?

It's the Micro 2000 Universal Diagnostics Toolkit. I got lucky and found it at goodwill for 5 dollars. Kind of ironic that the software that's supposed to fix bad sectors has a bad sector. 🤣

I might have a backup somewhere, but I will have to go searching through my 10000+ 5.25 inch floppies.

Today I have been messing around with my drives again and I have managed to get all 3 that I have working : Mitsumi D509V2, NEC FD1157C and Teac FD55GFR.

In the end the problem was combination of dirty cables, corroded jumpers, misconfigured jumpers and possibly some motherboard related things since this time I used a PIII board for testing rather than a 486. I took photos of all the jumper settings so that someone else with such issues may find a solution.

Mitsumi :

NEC :

Teac :

I'm not actually completely sure what all the jumpers on the NEC drive do, I tried to move some of them during operation and observed no change in behaviour...
NEC drive seems to have most make sense jumpers, they all do exactly what they say. That drive should be fine with 360KeB disks too.