Data transferred from a host system to magnetic tape storage are divided
into blocks that are converted, written to storage and read. An error
checker determines whether errors in each block exceed a predetermined
number. The result is output with corresponding header identifying
information. Information, including header identifying information
corresponding to each block written to storage, is stored in a history
store. Header identifying information output by the error checker is
compared with the header identifying information to identify the data
block relating to the error checking output. The marked data block
associated with the matching header identifying information is indicated
as being good or bad according to the error checker. Selected parts of the
header identifying information are masked to compare only the remaining
portion of the information with the corresponding history store
information.
