# File test/backup/DSK.rb, line 214
        def get_block(block_no)
                track=(block_no / 8).to_i
                first_sector=2*(block_no % 8)
                raise "illegal block no #{block_no}" if track>=self.track_count
                return self.get_sector(track,first_sector)+self.get_sector(track,first_sector+1)
        end