not listed in disk management
If you are certain that it is not listed in Disk Management at all ( and not merely present without a drive letter).
You may be running into something that I had an initial problem with, with respect to a couple external USB device when I first upgraded to Windows 10.
Without the external drive connected, go into Device Manager (right click the start button and choose it from the context menu).
Expand portable device and uninstall everything listed there with a faded icon (indicating that the device is not connected) by simply right clicking them individually and choosing "uninstall".
When you have those cleaned up then reboot your computer, attach the drive normally and see what happens.
Are you hearing the typical windows default sound when you are connecting and disconnecting USB devices?