FireWire drives (and other mounted volumes) automatically dismount when there are no logged-in users under Mac OS X 10.3.x. This is a change in behavior from Mac OS X 10.2.x, where drives remained accessible.
MacFixIt reader Mike Barron writes "At my lab I typically do unattended backups of key machines to firewire drives using rsyncx via crontab. (In Mac OS X 10.3.x) my scripts have not been running. After a few weeks of troubleshooting the scripts (some of my problems were related to the latest version of rsyncx) I ssh-ed into my work machine to work machine from home to see if the scripts had run. I cd-ed into the backup drive -- a firewire drive -- only to discover it was not mounted. I did some testing on my home machines, and, as it turns out, when no user is logged in to a machine, any firewire drives get unmounted."
One solution to this problem (not being able to backup to a volume on an unattended machine with no accounts logged in) by creating a new account with restricted access -- use the Limitations tab of the "Account" pane in System Preferences - and keeping it logged in at all times.