Undelete/unformat a Mac formatted external hard... I have accidentally formatted an 80gb external

Question

I have accidentally formatted an 80gb external Fujitsu(formatted for window xp)hard drive in my Macintosh OS Leopard, while backing by Mac's Time Machine. This hard drive contains family album containing thousand of photos. How do I undelete this Mac's formatted hard drive to get back my family album? Please advise urgently. Thanks a million. Rgds, Jay