My Passport works with MacBook Pro but not new iMac

I have searched this forum and other websites but cannot find a solution for why my WD Passport Essential works fine with my MacBook Pro but will not be recognised by the iMac, even with Disk Utility.  Have tried all the USB ports etc and the lights are on the WD but not being recognised.

If I plug it to the MacBook and then view it across the network from the iMac it works fine.

What is the solution?  I have read about some Y cable?  Or do I change to FireWire or will it never work for some other power reason?

Many thanks

Hello, the Passport might not be receiving enough energy to work correctly. Be sure to connect it directly to the Mac, avoid using any USB extension cables or USB hubs. If the problem continues, you can contact WD support directly and request a Y cable. Check the link below for more information.