PLugable USB 3.0 is not working properlyit either switches off and on repeatedly or doesn't recognise hard drives

Connected to iMac OSX 10.9.5
Unit is powered OK but when I connect external WD 1TB portable drive the blue light comes on above input but doesn’t appear on desktop. Drive makes clicking sound.
When I connect another WD 1TB portable drive no blue light appears & nothing happens.
When I connect Porche 3TB external self powered drive, a blue light appears but then the unit turns off & on continuously.
When I connect Seagate 2TB self powered drive nothing happens.


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