I used a 4 GB Sandisk SDHC with no issues at all. Check the link I'll provide for homebrew compatibility tests. You can make sure you try one already tested or take your chances with something else. Official Nintendo Wii cards are rebranded Sandisk's.
SD/SDHC Card Compatibility Tests - WiiBrew
BootMii/SD Card Compatibility List - WiiBrew
I used the 500 GB WD My Passport Essential USB 3.0. It works perfectly if you backup and install the software it comes with to disable drive sleeping. Turns on and off with the wii now with only 1 USB cable to fully power it. It only worked in the bottom USB port of the wii. 80$ is a bargain. USB 3.0 is supposed to be backwards compatible with USB 2.0 devices.
Last edited by tuscanidream; 03-18-2012 at 06:02 PM.