As everyone knows, the Wii U supports external USB hard drives for storing your didgital data.

What you may not know is that you may have an issue using a drive that is bus powered (ie, does not have its own power cord)

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Bus powered drives became more possible with the advent of USB 3.0.  The new spcification allowed for increased amp output which allowed for things like physical motors to be powered via the USB cable.

The problem is,USB 3.0 is backwards compatable with USB 2.0, but USB 2.0 provides limited current output for these devices.  The Wii U uses USB 2.0.  This can cause issues if the amp draw gets too high for USB 2.0 specs, effectively depowering the device, resulting in system instability.  

To resolve the issue, a Y-cable uses 2 USB ports, providing 2 voltage sources in parallel, doubling the ame output potential

Externally powered devices have no issue

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For most bus-powered drives, you will probably need what is called a Y-cable (not this is different from a Y-adapter, or splitter, so be careful)  Many drives will come with them, so if it did, no need to continue reading.

They are easy to find online, but usually run about 10 bucks, and are very easy to make for anyone who has ever stripped and taped a wire and has spare USB cables about.  I have made one for my drive and it is working perfectly

STANDARD DISCLAIMER: By making one of these cables, you are doing so at your own risk and are solely responsible for any damage caused by it

With that aside all you need is:

A wire stripper
Electrical tape
a USB cord with a type A end (standard USB end)

either:
A USB cord with a male type A end and a female type A end

or

A USB cord with a male type A end and an ened that can plug into your device.

I was going to make a second and take pictures and whatnot, but I found someone who already did that.  So just click below:

(OVERVIEW: Strip back the casing of the Male-female cable, cut red and black, connect red and black from 2nd cable, tape, done)

http://xkeydownloads.com/tutorials/Make_a_Y-Cable.pdf

How can I tell if my HDD needs to be Y split or not?

http://www.amazon.com/gp/product/B00834SJSK/ref=oh_details_o00_s00_i00

Got this in the mail today, plugged it in, formatted it, and now I am currently transferring my downloaded data to it. Everyting seems to be working fine. How do I know if it's incompatible? When I try to play a game?

The thing is not that it wont be compatible, it is that you cannot gurantee that the drive is getting enough power. It could work just fine for most things but fail to do so while playing games (due ot the higher amount of power needed) , or it could just work fine all together, there is really no way to know except to try it and see. The device will certainly work in  all situations it is designed to if you use a Y cable though. 

Actually I have the 1TB version of the drive you linked, and I had to purchase a ycable to get the drive to work with my 360 but it worked fine with my PC (I assume this is becuase the USB ports on the 360 do not give as much current as the ones on my PC)

Why would I do all that vs just buying one for a few bucks off of internet? or better yet, buying a HDD with an A/C plug in the first place.

Hrm.. well.. it appears to be working fine. Let's say I start playing NSMBU in a few minutes, how will I be able to tell if the HDD stops working? Will it automatically throw an error sign, or what? I don't really expect you to know, but I'm just rather confused.

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Well if you are running the game off of the HDD and it is not getting enough power it would likely just crash and or start lagging really badly. While I don't know what happens with the Wii U exactly (mine wont be here until next Tuesday ) that is what happened when I attempted to play HD videos off of mine on my 360 before getting a Ycable, now they play fine without freezing or lagging.

Edit - How do I delete posts? I accidently replied at first without quoting you. 

The reason that I did was because I already had a 500MB drive that ended up needing one, I already had a bunch of USB cables, and am impatient :)

If you have to wait for a drive to ship anyway, then there is no point in doing this

What will happen is you will get an error saying that the drive was unplugged.  What happens is it starts to draw too much current and shuts down the connection, effectively disconnecting the drive.  Unfortunately when it happened to me, I had to unplug the Wii U to get it to shut down.  It also caused Nintendo Land to freeze when loading, probably because it was in the middle of looking at the drive. 

If you did a lot of data transfer and had no issue then your drive may not need one, it may operate within the USB 2.0 power limitations.  However, if I were you, I would pick up a cable just in case

Questions:

1) Your link says that only works with USB 1 and 2.0 cables. I have 3.0 cables... does this make a difference? Can you get me a new guide?

2) Can I mix a 2.0 USB cable and a 3.0 USB cable? I don't seem to have any extra 3.0 cables around...

*Edit* Skipped the hassle and I'm just going to wait a week for newegg to ship me one.. Oh well.