Hard Drive Problem? - Making a Copy of a working External HardDrive!

So to cut a long story short.

I have been using my Wii U for homebrew Mega Drive, Gamecube, Wii Games etc. with no problems. However, despite having a Y-cable to power the Hard Drive, it would often freeze due to lack of power.

To prevent this I bought a 2TB Seagate Expansion Desktop, which is self powered. Now I have noticed that this is recommended by Nintendo.

I formatted the drive to Fat32, which I believe is the same as my old drive, and I copied a few of the files across to test if it works. I have also tested with NTSC.

I am still using the Y cable and hard drive is also plugged into the power socket. I enter the VWii and click on the Homebrew Channel. I get nothing but bubbles. Now on my original drive all my apps would appear straight away. I figured out that if I press on I can select USB or SD. I select USB and everytime I get the black screen titled "Exception (DSI) occurred!". I have to unplug the power from Wii U at this point. I can't figure out what the problem is.

Any help would be much appreciated. I just want to make a copy of my original hard drive.

Thanks In Advance.