FireWire external drives are, unlike USB, very sensitive to how they are terminated from your computer. The device icon must be hi lighted, and dragged to the trash, (or menu-file-eject), before it is turned off or unplugged.
My suggestion for trouble free use of your external is to keep it powered off and disconnected from your Mac until you need it. At that time: power up device and wait a few seconds. Connect device to Mac. The device icon will show up on your desktop, through which you can access the drive. When through using HD, hi light icon and terminate device. Power off, and unplug HD from your Mac.