Error #73:   DOS MISMATCH (73, CBM DOS V2.6 1541)

  DOS 1 and 2 are read compatible but not write compatible. Disks may be
  interchangeably read with either DOS, but a disk formatted on one version
  cannot be written upon with the other version because the format is
  different. This error is displayed whenever an attempt is made to write
  upon a disk which has been formatted in a non-compatible format. (A utility
  routine is available to assist in converting from one format to another.)
  This message may also appear after power up.