Firewire drive intermittent problem

[toyfactory]

Usually Warty has no problem detecting my firewire drives.  However
sometimes (one in ten boots perhaps) I receive an "unable to mount the
selected volume" error because mount reports that the device does not
exist.  There's no pattern; sometimes it's when I first turn the
computer on, sometimes it's when I reboot.

Any ideas why they don't get detected and how I can eliminate the
problem?  The easiest answer of course would be to leave my computer
running, if I weren't concerned about wasting energy.

Nick