If cbmformat is used to format a disk, the disk is
unusable, has read/write errors, or shows other
unexpected (and unwanted) behaviour.
Andreas Boose found the reason: cbmformat is highly
picky on the RPM of the drive. If the drive runs at approx.
280 rpm (or lower) instead of the (nominal) 300 rpm, it
overwrites part of the track it has just written.
This bug will be fixed in the future.