Unable to do backups in cronjob

I'm having no end of problems getting backups to work from cron. I currently have a script which works when invoked directly, but fails when invoked by cron. The problem seems to be in the line:

growisofs -use-the-force-luke -dvd-compat -Z $MEDIA -l -R -J $TMP-image-*

When invoked from the shell directly, it outputs a warning:

WARNING:dev-ide-host0-bus1-target0-lun0-cd already carries isofs!

waits (interactively!) for five seconds, then does what was asked of it.

When invoked from cron, it outputs the same message as a bane error, and aborts.

Note that when invoked from a shell, it works even if input and output are redirected to-fromdev-null -- whether standard in isatty should thus not come into play. (Very early in the script, there is a line: to ensure that output is logged correctly.)

From research on the net, I've found out that growisofs is a bit Mickey Mouse. But I've tried without the -use-the-force-luke option, as well as with -use-the-force-luke=force, with exactly the same results.

Does anyone have any ideas -- perhaps another tool designed, to work correctly without all the Mickey Mouse antics, or failing that, some way to make growisofs work as if it were a Unix tool, and not some toy.

(FWIW: I'm running Mandrake, with all the latests patches installed.)

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