If the external drive shipped with factory firmware and software, such as Western Digital, or Seagate management software, you may need to install or update the drivers and software for the external drive.
If you mean the drive isn't visible in Explorer; try assigning it a drive letter. Open Disk Management and right-click the external hard drive, select Change Drive Letters and Paths from the context menu, then assign a drive letter.
Sometimes USB devices and Windows just dont operate correctly. Disconnect the external drive, then open Device Manager and uninstall the drive. Reboot the computer, reconnect the drive.