You run into that a lot, or i should say that i do. I have about a dozen or so usb\firewire drives, some are external boxes, some thumb drives.
My firewire box with a maxtor 80 gig h drive in it does the same thing. I cant use the "safely remove hardware" button in my tray for this unit, it just wont work.
Usually i just turn it off and all is well. If i have been writing to it lately then i will go to task mgr and log off and log back in again and then it will turn off. But doing that so many times i figure its about the same as just waiting a while after the last write and just going ahead and turning it off. You arent supposed to do that but what the heck can one do when it wont work the way its supposed to!!
Going to properties and turning off the "enable write caching on the disk" is supposed to help but it hasnt helped me any with this one unit, might help you out though. I believe this is also called "write behind" as well.
My other units will turn off using the "safely remove hardware" icon, just this one firewire drive wont. Sometimes they take more than one try but eventually they turn off, except of course the one firewire unit.
Good advice + great people = tek-tips