You should maybe try on another computer how it is detected.
I don't think it's because your HDD was not in Fat32 before WBFS, it seems more a problem of Usb Drivers if the HDD is not detected.
If when you launch Active@ Partition Manager (a freeware) it detects your drive on the list, it seems more of a WinXP problem. I'm not sure if you can try to mark the WBFS partition as active with this freeware, but if yes it could help with the
One more reason to always make 2 partitions on Wii Hdd : one WBFS and the other NTFS (or Fat32).