I used to have issues with using an external drive for sharing. It seemed the external drive was intermittent in communicating with the OSX system & didn't always register with LW at the time LW was opened. Whether this was due to the firewire 4 or old speed system bus on my old G4 I don't know. It might also be an issue with how OSX handles external drives.

My suggestion would be to delete the LW preferences folder; Deleting LW Preferences folder in OSX

I know this is an old thread, not sure why it was not answered earlier.