When you don't see the disk in Explorer, does it appear in device manager? In windows 7 right-click Computer in the start menu and go to Properties, and select Device Manager on the left hand pane or in 8.x right click in the bottom left corner and select device manager from the menu of options. If it's not being seen at all there may be a problem with the drive's controller. Check that the SATA cable is secure and try it in a different SATA port. If these don't work you may need to replace the drive. Any data on that drive you want to keep should be backed up to an external drive just in case (this is the best policy in general, even if there's no sign of imminent trouble)