My System is a Dell Precision Tower 5810 running Windows 8.1 Pro 64-bit.
I have a couple of older WD My Book Essential External 2 TB Hard Drives.
This is the WD Drive before the current My Book came out.
When I connect either WD My Book Essential External 2 TB Hard Drives to the 1 USB 3.0 Port on the front of the Tower, my System does not recognize that it is attached.
When I instead connect it to either of the 3 USB 2.0 Ports on the front of the Tower, my System does recognize that it is attached.
WD told me to try a USB 3.0 Port on the back of the Tower.
I did that & Viola, my System now recognizes that it is attached.
Since the WD My Book Essential External Hard Drives do support USB 3.0 I would of course rather use a USB 3.0 Port for the additional speed.
But, there are only 3 USB 3.0 Ports on the back of the Tower and I use all three (2 Monitors and the UPS).
I had to disconnect my UPS to be able to use that USB 3.0 Port.
I researched my problem and there seems to be conflicting opinions as to whether or not it matters if you use a USB 3.0 Port on the front or the back.