You probably need to hook it up to the Mac, open Disk Utility, select the FW drive, go to the Erase tab, set Volume Format to HFS+ (or MacOS Extended), and hit the Erase button. This will reformat the drive for the Mac (make sure you've backed up anything that's on the drive).
There are ways to make drives work in both Mac and Windows but I'm not sure how to do it. There's a utility called MacDrive but I don't know if it's recommended or not.