One possible reason could be virus. Virus would have corrupted or infected the second hard drive. So it is not recognized by the system.
Second possible reason could be the disconnection of cable. So if the cable connecting the hard drive might have been worn out. So try changing the cable.
Third reason could be the power supply. Power supply might not have been received by the hard drive, so the hard drive might not be switched on and hence it is not listed to the user.
