DSNU543I csect-name - I/O ERROR READING PAGE page_number IN THE PRIMARY COPY DATASET dataset-name, BACKUP COPY IS USED

Explanation

If RECOVER or MERGECOPY has an I/O error when reading a page, with the specified page number, from the primary copied data set, it will locate the backup copied data set in that same system (local or recovery) and continue reading pages starting from the page number in error.