> ncr5380_command: short transfer: wanted 10 got 6
> si(0:0:0)-28,0,0,3,b1,0,0,0,10,0-

This happens when command phase doesn't last as long as it should.

Commands can be 6, 10 or 12 bytes long; the first byte indicates the
length of the command.  In your case, the command is a read(10) - a
10-byte READ command, so NetBSD was right in expecting 10 bytes.

Perhaps the MD21 always does this for a read-10 command.  You might want
to look in sd.c to see when 6-byte commands are used if ever.