cpio may report an I/O error:

I/O Error on output .

A detailed overview of the possible error messages can be found in the description of message BR278 in Messages or in the SAPNet Notes database, if you enter cpio as selection criteria.

If this error only occurs sporadically, you should check whether it might be related to the use of a specific volume. If so, you should no longer use that volume for backups.

If the error occurs with different volumes, you should have your backup device inspected, as a hardware problem is probably to blame.