I tested the external drive at windows machine at work and it has was exact same problems. One floppy did not work at all, second was trying to format but after couple of seconds ends with error. They are 20 years old. On Linux ufiformat gets to the end.
I will buy a new pack of floppies (not so easy to get those days). And then if all fail I will buy another external drive (new this time).
Summary on Debian Linux (I tested both /dev/sdc and /dev/sg2, doesnt metter):
$ sudo ufiformat -i
disk generic
/dev/sdc /dev/sg2
$ sudo ufiformat -i /dev/sg2
vendor: MITSUMI
product: USB FDD
write protect: off
media type: 2HD
status block size kb
unformatted 2880 512 1440
$ sudo ufiformat -f 1440 -v /dev/sg2
format on device=/dev/sg2, size=1440
geometry: track=80, head=2, sector=18, block=512
done
Everything looks good but when trying to mount:
$ sudo mount /dev/sdc /media/floppy/
mount: /media/floppy: can't read superblock on /dev/sdc.