The ftape --driver for using the cheap floppy streamers is not very reliable. This is not a problem of KBACKUP , but of the driver support in the kernel, or, more precisely, of the poor design of these devices. If you can afford to choose, get a streamer with a proper interface, like SCSI or maybe an old QIC-02 streamer.
For more details on using floppy streamers, see the Ftape-HOWTO.