If Pro Tools gives this error only when the Firewire drive is plugged in, you may need to format and/or partition your drive- there may be an error in the way it was formatted. Or, try going into Device Manager in Windows 98 or ME (Right-click My Computer, Properties, Device Manager, Double-click Disk Drives, Double-click the drive (should be listed as some sort of Firewire, IEEE 1394 drive, or the drive's actual name), go to Settings, and uncheck Removable and check INT13. Then reboot your computer. This might help your drive to show up in Windows correctly.

Also, you should find out if the drive uses an Oxford chipset to convert the Firewire into the hard drive. This is the recommended chipset.