Simply the OS is unable to read the external drive. First make it read, by plugging it on different ports(front/back) USB2.0/3.0.
Try it on safe-mode.
Try it on another system, or another OS, see if it gets detected there.
Try it numerous times but properly unmounts it.
Try to uninstall the ex drive from Disk Management, then Scan for Hardware Changes under Action menu. Good luck.