The BK (an abbreviation for 1ba9 is a Soviet era personal computer from Elektronika based on a PDP-11 single-chip processor. It was the only official, government approved home computer in mass production at the time.
It got a floppy interface in 1989 when the 128kB BK-0011 was released. This used a relatively normal double-sided IBM scheme format with 80 sectors and ten sectors per track, resulting in 800kB disks. The format is, in fact, identical to the Atari ST 800kB format. Either 5.25" or 3.5" drives were used depending on what was available at the time, with the same format on both.
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To read:
fluxengine read bk -s drive:0 -o bk800.img
To write:
fluxengine write bk -d drive:0 -i bk800.img