well, it is laptop, so only USB now.
now i narraow the problem to the format of the diskette. I can read the diskette and see the correct files in linux after creating it. But when I access the diskette in windows, all file names are strange stuff. So, maybe computer just can't read it to boot...
Weird. I add in etc/fstab the line
/dev/sda /mnt/floppy vfat noauto,user 0 0
mkbootdisk --device /dev/sda 2.4.18-14
Why this works with another linux box with an ordinary floppy?