I had the same issue with my 320GB Acomdata FireWire Drive on my I-Mac. To open, you have to pry the aluminum plate off of the switch end of the drive which is hiding the four screws holding the drive inside. Use a slim flat blade screw driver to wedge between the plate and black plastic and the aluminum plate it will come off. Why Acomdata hid the screws is anyones guess.
I installed my drive into a USB enclosure and tested it and it worked fine so I scanned it with Spinrite drive recovery software (well worth $79) and the drive checked out fine. Reinstalled it into FireWire enclosure and drive worked flawlessly.
Hope this helps.