If a Backup abends while running with sysparm SYNCDEVS set to N99 (the default) a discrepancy between data sets written on tape and DSNINDEX records added to the CA Disk Files Data Set can occur. How to recover from this?
Just rerun the BACKUP and MERGE the output tape from the abended Backup.
Merging the output tape from the abended Backup would only forward the backup copies on the tape with corresponding valid DSNINDEX records, but it would create a correct tape with matching DSNINDEX records.
So in any case a subsequent MERGE will correct the mismatch between tape contents and DSNINDEX records based on the existing DSNINDEX records.
Otherwise, to determine which data sets are already on the tape, run the Tape Analysis Utility as described in the CA Disk Systems Guide and compare the listed data sets to the result of a LISTD VOL= for that archvol. Then run a REBUILD to create the missing DSNINDEX entries.
A REBUILD will correct the mismatch between tape contents and DSNINDEX records based on the tape contents.