What version of ROMWriter do you have?
Go here, http://tech.prolific.com.tw/visitor/v_welcome.asp, you'll have to click on download. Then, under the USB/1394 IDE Controll, choose the PL-3507 link. Choose the firmware category from the drop-down box. On the next page, it will have a few differnet ROMWriters and firmwares. You will need to download two of each most likely. You will first need to use ROMWrite 2.0.4 to change to firmware 092204, then use ROMWriter 2.2.1 to upgrade to 120904. You have to do this over USB....you cannot do it over firewire. It will not work, if I remember correctly.
There is also the Big External Enclosure thread in the main Recording Hardware Forum. I recommend checking it out....there is a lot of good info there, and more people to help with any problems.