WD My Book Studio WDBCPZ0040HAL vs WDBHML0040HAL

I have tried unplugging it and replugging it back in and using both Firewire 800 cables and USB 2.0 there is power but the drive just won’t mount. ANy help is much appreciated I’m running OSX Mavericks. Another duplicate drive runs fine but sadly it doesn’t contain the files I need