NEVER use your main system drive to edit video from it is already working hard running the operating system and any application that are running making it access video at the same time is just askiing for poor performance and lots of droped frames in video playback while editing.
It is preferible if you have a second internal drive to edit from the data transfer speeds are faster, but a firewire drive should work fine. DV and HDV video take up rufly the same amount of drive stace and same data transfer speed so ther should be no reason that they wont work well from a firewire drive.
If FCP crashes every 30 min I dont think it is a problem with the drive most likley FCP Trash your preffs.
You said that you unchecked the auto save if FCP keeps crashing you will want that turned on so you can go back to the most recent auto save after a FCP problem.