I had an extra 500 GB WD Passport drive that was my old Wii drive.
I cleared that off to use it in the U, but apparently it won't work without a Y cable, even though it self-powers fine in my Wii and my PC. I don't love the fact I need to take up both USB ports for it, so I might switch to the SD card method.
I have a spare 200 GB one I might use temporarily until I find out I need a larger.
So I quickly realized I was going over 200 GB on my games, so I've gone back to my 500GB Passport drive.
I've got another Wii with a 2TB so if I want to play Wii games I can do it there. I've injected a few Wii games into the WiiU and with the exception of one which doesn't work with the mic, they all seem fine. This means I can just use one drive for both WiiU and Wii games so taking up both rear ports isn't a huge deal right now.
It seems that SSD drives work best when self powered through one port, so if I didn't care about $$, I would probably get a 512GB-2TB SSD for each port. While the injection of Wii games works, I would almost rather the organization the USB Loader GX gives you rather than have them all shown on the main launcher.
While I usually spend a lot more time, thought, and $ getting things how I want them, this was a hand-me-down console and I never really get a lot of playtime in anyway, so I think for my casual usage it will probably stay this way for the foresee-able future.
I do want to also note some informal performance statistics. When using the WUP installer to install games to my 500GB WD Passport Essential (5400 rpm) drive I am getting transfer rates of about 1GB/50 seconds. The same transfer to my SD card (SanDisk 200GB Ultra microSDXC UHS-I/Class 10 - SDSDQUAN-200G-G4A) took about 80 seconds I believe. I forget the exact numbers, but the HDD was definitely about 50% faster than the microSD. I know there are some better class microSD cards out there, but they are pretty pricey. I would be interested to hear about speeds to other cards.
I will put my 60GB spare SSD back in and test again to post some numbers.
Also, I will try to launch a game or two and see if I see significant differences in launch time. After all, you tend to copy games just once, so more important is the read time off these.