External HD problem after formatted

I had format my WD external HD because it was infected by virus.Unfortunately,the fle system is not NTFS file anymore,the status is healty,the capacity is still same like before 298.09GB,fault tolerance,%free and overhead is almost ok..the only problem is the file system is null even I had set to ntfs when I do formating.can you give me solution..I love my HD so much because the capacity is quite big 320GB..