During backup command processing, an error occurred in updating the data set VTOC entry for the data set being backed up. This message is either preceded by message ARC1001I or referred to by an associated ARC0734I message. Both messages include the name of the data set being backed up and a reason code. If the reason code is 0, an unrecoverable I/O error occurred and the DCB SYNAD exit was taken from the data management CHECK service routine. If the reason code is nonzero, a request to the common VTOC access facility (CVAF) to read or write the data set VTOC entry failed. The reason code is the contents of register 15 on return from CVAF. An associated hardware, data management, or system error message can be found in the DFSMShsm job log SYSMSG data set.