Phil,
 
    EXECIO  return codes  in the  1xx  range are  from underlying  CMS
services such as I/O macros (CMS Command & Macro Reference, pp.  146).
From what  you say,  I'd assume  it's an  EXECIO DISKW  failing, which
means the return  code comes from FSWRITE.  RC=13 means  disk full (cf
pp.  540).
 
    Have you done a QUERY DISK N lately?
 
Ross