Unable to make a backup of Win 7 on Mac book

I could not make a backup for my Windows 7 partition on the OS X Lion pre installed on my Mac book. After the recent installation of Windows 7 on my Mac book pro 13” laptop, I connected the external hard drive and back up the Windows 7 but it wasn’t possible because of the format. Are there instructions or a procedure of formatting my external hard drive so that I can still back up Windows?
