If you have ever connected 2 identical ext HDDs and found both work fine independently but not when both plugged in.
Then you may have this unusual problem I just struck.
Bought WD Elements 500GB as a present 3 months ago.
Bought another on special recently.
Whenever one is plugged in first works fine. Second one is not assigned a letter and offline. Reverse the disks and the same applies.
Both where manufactured with the same disk signature!!!!
Changing the disk signature of one solved the problem.