Well I didn't see a thread so I thought it was worth starting.

List your experiences of what works and doesn't work.

I'll go first:

I got a Imation 320GB Apollo M100 Portable Hard Drive 2.5inch.

When I first connected it my Wii U wasn't picking it up. So I googled and noticed that some people said it was not compadible.

What I did however was go to system setting > data management to see if I could format it and I could not. Then I exited data management back to main menu, then I decided to go back into data management and noticed the drive is recognised by the Wii U.

It works great once recognised.

Here is the twist, every time I power down the Wii U and then later power it back up to play more games it does not pick up the hard drive. Once I go to data managment menu (doesn't show up at this point) then exit back to main home screen it recognises it and my demo icons appears.

The reason I am writting this is because I also wonder how many other portable drives would work if people tried this trick without the need for a Y cable.

So has anyone else experienced the need to go into data management and exit every time for the hdd to show up? bug in OS? 

Good idea!

So far Nintendo hasn't been said much about which ones are compatible or not, the list is quite short.

That sounds very frustrating. If I get a hard drive for my Wii U, it's because I want quick easy access to my games. Boot, tap, play. I wouldn't want to go through menu options.

Nice observation! Do we have an official thread for this topic yet?

One thing I really want to know is if we can use SD Card + USB plug combos to save our games on? Nintendo's official site says you shouldn't use USB sticks because they're not made for constant data access. But what about SD cards? There's one in my 3DS so there shouldn't be a problem if I use an SD to USB adapter, right? Having an external HDD next to my Wii U isn't a very sexy solution.

I've got an old 500GB desktop-class LaCie with its own power supply. Works well so far. Seems faster than loading from disc, perhaps not as fast as internal storage.

One annoyance is that transferring games to the HD is all or nothing. I'd like to keep the irreplaceable save data on the console itself (I don't entirely trust this old drive), but I don't seem to have any choice but to transfer it to USB storage along with all the other game data.

I have a 500GB seagate HDD attached to mine.

Be careful if you have a USB 3.0 HDD, you will need a Y-split USB cable in order for the Wii U to power it (this means plugging the HDD into 2 USB slots simultaneously). My HDD worked with just a single slot, but during gameplay it would freeze so I had to get a Y split. Sucks, but oh well.

What games do you have on your HDD? I'm thinking about downloading Monster Hunter because I'm too lazy to swap discs all the time. But apparently the internal storage is quite slow (and a lot of HDD's are too) so I'm afraid loading times will actually be worse.

I really wish it had interanal hard drive of more than 32 gb >.> Come on nintendo it's not 2005!

I have a hard time believing any kind of flash or hard drive could be slower than an optical disc of the same generation. I'm not really inclined to buy the same game on disc and download to do an apples to apples comparison of which loads faster, but I might do some testing and see if that gives us an idea. Certainly I can at least test loading times between internal storage and an external hard drive.

Oh, and I do currently have Monster Hunter on internal, so it should make a great test case, especially with all those load screens.

So to fully understand this.

Once you have a HDD, absolutely nothing is allowed to be saved on the internal memory? I'd rather have game saves on the internal memory as well with full games on the HDD.