Answer by  
                                                                                          Mercurious (18)
                                                            
                          
                          
                                                          Possibly because the external-HD is formatted in a way that Windows 7 doesn't naturally read. To check this use the Computer Management tool which is found in Start | All programs |  Administrative Tools. Try in Accessories if you can't see it.
 
 Once open click Storage|Disk Management in the left explorer panel to see if your disk is visible.