I've just changed the formatted tracks to report ND, while unformatted tracks report NID instead(also applies to wrong rate and motor off).

Verified same behaviour on MS-DOS. Also same behaviour on Windows NT 3.1. And also the very same behaviour on Windows 95.

I've noticed that both MS-DOS and Windows 95 seem to misdetect that a track isn't formatted. Windows NT 3.1 seems to detect that properly.

Also, I've noticed that both Windows 95 and Windows NT seem to not properly change the disk when it's changed while the disk motor is still spinning? Is that normal behaviour? Windows 95 even seems to request, using a BSOD-style input, for the previous disk? Windows NT displays the previous disk's volume label, then the new disk's content in that case?

All those errors reporting actually set both ST0's Unit Check(bit 4) and Not Ready(bit 3). Anyone knows what the correct ST0 value for such errors is for those two bits? Or are those two always cleared when ST1 bit 0 or 3 is set with the case of not found sectors(ND), not found index of the track(NID)?

OK. The first effect I notice when removing the ST0 Unit Check and Not Ready bits is that MS-DOS format.com can actually format the IMD disks without requiring the unconditional format /u parameter now, which is a nice improvement. Executing dir on the disk still results in a "General failure reading drive" error message, though, so at least the message doesn't change.

Edit: Windows NT performing "dir" on the unformatted floppy disk says it's "not formatted properly". Format.com reports it's filesystem is "RAW". Formatting succeeds and the disk is usable.
Edit: The same error message as MS-DOS 6.22 gives is given by Windows 95 when the disk isn't formatted. Perhaps it has something to do with being based on MS-DOS 7 and 6.22 respectively that's being run?

So now, the only problem that's left is the response by MS-DOS 6.22 and Windows 95 when a IMD floppy disk is inserted, but isn't formatted. The format.com program handles it without issues now, but the dir command gives the "General failure reading drive" message? Why doesn't it give a more descriptive message like Windows NT does?

Edit: After reading https://www.pcmag.com/encyclopedia/term/gener … reading-drive-x , perhaps it's supposed to do that when the disk isn't formatted? So the FDC is fully functional now?