Normally, the Wii normally won’t allow SD Cards larger than 2GB, so larger cards such as the typical 4GB variety are completely useless for storing all of your games and save information. It turns out that with a few tricks, you can fool your Wii into accepting these larger cards.
The hack, documented in detail by Knoxximus over at cheapassgamer.com, basically amounts to filling up the card with junk data until the remaining free space is less than 2GB. You first use a PC with the SDFormatter program to initially format the card (since the Wii won’t format the 4GB card to begin with). Then you copy some files to the disk using your PC, bringing the free space below 2GB. Then the disk magically works in the Wii.
Though the filler data takes up usable space on the disk, you can remove filler files as the disk fills up. Essentially, you can access all 4GB of the disk – you just need to ensure that free space at any point in time is less than 2GB. Knoxximus has a PC utility that will create a bunch of files on your SD Card with a user specified size, which makes this whole process fairly simple. For instance, you can make a folder with hundreds of 100MB files, and you just delete a dummy file or two whenever you fill up your card with Wii files.
It sounds like the 2GB limit is completely arbitrary, since writing to a 4GB disk works without problem. Somewhere in the Wii’s software, it’s just checking to make sure there isn’t more than 2GB available on the drive. I’m not sure why the developers didn’t check the volume size of the disk instead of the just the available space, but thankfully they didn’t or this hack wouldn’t be possible.