_Rob wrote on 2020-05-12, 19:12:

Your however using /dev/sdb, which to me means it is not a real floppy drive. But probably a USB Floppy Drive, which uses a completely different protocol called UFI.

You are right. It's acttualy TEAC. FDC controller is embedded inside and system has no access to low-level floppy functions. That's the reason why I expected IMGMOUNT just to work. System itself isn't aware what is in that block device. It's just regular 1,44MB/512kb-block disk. Coincidence.