Why?

Even though the Wii U has a built in SD slot, it doesn't support using it as a storage expansion to store Wii U games (unlike it's predecessor). USB pen drives are notoriously unreliable and hard drives are bulky and require extra power or a Y cable, using up multiple ports. Today big reliable SD cards have become cheap. Since a SD is needed anyway for homebrew, it would be nice to use that too as storage for games.
There have been solutions in the past like Loadiine, but this had various problems, the biggest of them performance and is not longer supported by current homebrew environments (Aroma).

SDUSB

SDUSB solves these problems. It uses a second partition on the SD card, which will be formatted to the Wii Us native file system and therefore run at full speed. The partition will show up as a USB device and can therefore be managed using the built in Data Management in the system settings. SaveMii, WUP Installers etc. all work with this, like it is a USB storage device. Also HAI (VC Wii Titles) work with SDUSB.
SDUSB does all that by patching IOSU (the OS that runs on the ARM processor). It is implemented as a stroopwafel plugin.

Prerequisites

You need two things:

1. a way to launch minute
2. a reliable SD card

For 1. the recommended way is to setup ISFShax, for that we have a guide here: How to set up ISFShax
If you don't want to commit to installing ISFShax yet you can skip the "Installing ISFShax" step in the ISFShax setup guide and instead run it manually through the chosen exploit on every reboot.
Instead if ISFShax you can also use defuse, in case you have that already.

For 2. It is highly recommended that you use an Endurance branded SD card from a reputable brand. Since your save games will also be saved there, you rather want to spend $5 more then to lose all your save games because your cheapo sd card died. Also be aware of fakes, even on Amazon you can get fake SD cards...
The speed of the SD card isn't too important, as the Wii U is limited to 25MB/s (same as the internal memory) anyway. Every somewhat recent SD card should be able to get that speed. Choose Reliability > Access Time > Throughput.

Setup

Partitioning the SD card​

On Windows you need to use a third party tool like Minitool Partition Wizard or Easeus, on Linux you can use gparted.
You need to have two primary partitions on the card:

1. FAT32 - (in gparted set lba flag). This is what the PC will see and all your homebrew goes (you should already have this)
2. NTFS - This partition will be the "USB", you use to store the Wii U games on (don't assign a drive letter)

Shrink the existing FAT32 partition to make room and then create the primary NTFS partition after it. It's recommended to align the Partitions on 64MiB boundaries and use a multiple of 64MiB for the size. NTFS won't be the file system the Wii U will be using, it is just there to tell SDUSB which partition to use (it will pick the first NTFS one). The Wii U will later format it with it's own file system.

​

Installing the Plugin​

Get the latest wafel_sd_usb.ipx from here: https://github.com/jan-hofmeier/wafel_sd_usb/releases and place it in your ios_plugins folder. That is either wiiu/ios_plugins on the SD card or /sys/hax/ios_plugins on the slc. For slc you have to rename it to something shorter like sdusb.ipx

Using SDUSB

If you now boot boot back up, the Partition shows up as a USB device, which needs to be formatted and can then be used as usual. After formatting the SDUSB, you can also connect an existing USB storage and copy stuff over.

Known Problems

• GC VC Injectes don't work when installed to the SDUSB (they still work from Internal Memory)

 

[peilezhang]

What do you mean with Aromas second menu?
Also the vWii isn't affected by this. But I think most Wii homebrew already has USB support? Not sure.
The only potential problem I could see if you run a Wii VC homebrew injects from the WFS partition and that tires to use the FAT32 partition

This is the first menu

And below is second menu
When my USB device has only one partition (fat32),I had to wait over about 30 seconds to see this second level menu show up.When I re-divided this USB device into two partitions (one fat32 and one ntfs), the waiting time became normal(about 1s).

 

[Blythe93]

The first partition gets mounted as SD card to the Wii U, if a NTFS partition exist, that wiull be mounted as USB (wfs) storage.

That's how I formatted my HDD and, as expected, it asked to format NTFS partition and Aroma booted without issue. Wii U game (Art of Balance) works without issue. Wii VC title (Super Paper Mario) also work!

Since a HDD takes longer to spin up, it would be interested if that still works for cold booting aroma.

From what I can see, it works for me. I've ejected the SD card and have only HDD plugged in. IOS plugins and minute are on SLC.

If both a sdcard and a FAT32 partition is present the SD wil be /dev/sdcard01 and the USB FAT32 will be /dev/sdcard02. For some reason Aroma still uses the USB one.

So, I should insert the SD card and have HDD connected at the same time?

Anything else that I should test?

 

[SDIO]

@peilezhang does this also happen without aroma and with an older version of aroma (from before the last big update)

@Blythe93 it will always use the fat device that was last connected. If both are connected during boot the USB one is the last one which is initialized and therefore win. This seems to be a limitation the the FAT driver that the Wii U uses. It only seems to support one device.
I wouldn't recommend having both connected as that may lead to corruption (but it shouldn't)