So here the Y cable I'm thinking of buying it's to connect my Western digital Elements HDD to a different power-source rather than using the Raspberry Pi as an only usb power but also using the other USB to allow for my Pi to mount the HDD, will it work?
That is going to back feed power to whatever it is attached to.
I purchased a powered USB hub from eBay that is then plugged into my Rasberry Pi2. I have a USB2 sata docking station with a NTFS formatted drive in it that plugs into the hub, THEN into the PI. I find that my pi runs very stable with a low load on the built in hub, and a heavy load on the powered hub. I'm now looking at purchasing a USB3 Dock for flexibility with other laptops.
This is the unit I purchased: USB2 7 Port powered hub pi compatible