EXIT STATUS
       The exit statuses of smartctl are defined by a bitmask.  If all is
       well with the disk, the exit status (return  value)  of
       smartctl is 0 (all bits turned off).  If a problem occurs, or an
       error, potential error, or fault is detected, then a non-
       zero status is returned.  In this case, the eight different bits in
       the exit status have the following  meanings  for  ATA
       disks; some of these values may also be returned for SCSI disks.       

       Bit 0: Command line did not parse.
       Bit 1: Device  open failed, device did not return an IDENTIFY DEVICE
              structure, or device is in a low-power mode (see '-n'
              option above).       Bit 2: Some SMART or other ATA command to
              the disk failed, or there was a checksum error in a SMART
              data  structure  (see '-b' option above).
       Bit 3: SMART status check returned "DISK FAILING".
       Bit 4: We found prefail Attributes <= threshold.
       Bit 5: SMART  status  check returned "DISK OK" but we found that some
              (usage or prefail) Attributes have been <= threshold
              at some time in the past.
       Bit 6: The device error log contains records of errors.
       Bit 7: The device self-test log contains records of errors.  [ATA
              only] Failed self-tests outdated by a  newer  successful
              extended self-test are ignored.