I suspect the USB FDD standard does not support telling the device what type of disk is inserted. This supports far more than the IBM compatible disks that can be determined just from the notches.
The notch could be done to trick single-sided drives into letting you flip the disk over, which is probably what I was thinking of, but TFA says most drives can't read those with this controller.
According to wikipedia 40 track diskettes could be read by most 80 track drives, but once it was written to by an 80 track drive it was typically no longer readable by a 40 track drive.
I remember our first computer (a 386SX16/20) which my Dad bought sometime around 1989/1990, started out with two 5.25 inch floppy drives – a 1.2MB drive and a 360KB drive, so we could write 360KB disks without corrupting them. (My Dad thought this was important, I guess it was somewhat back then.)
Soon he decided we needed a 3.5 inch floppy drive as well. But he didn't want to give up having both the 1.2MB and 360KB 5.25 inch drives. The existing floppy controller only supported 2 floppy drives, so he went and bought a new one which supported four floppy drives. The chassis had enough drive bays to fit all three floppies and a 40MB hard disk. We had to make some changes to CONFIG.SYS (maybe an installable device driver??) since BIOS/DOS couldn't detect more than 2 floppy drives by default.
My first computer was a decommissioned CAD machine from my dad's work. A 12.5MHz 286. It also had two 5.25" drives in that configuration, but never thought why. Eventually installed a 1.44MB drive in place of the single density drive.
