It works now!
The problem was, that I initially formatted my HDD using Wii Backup Manager. It somehow seems to break something. I had no problems using the drive on Windows or with homebrew (other than WiiMC), but as soon as I tried to use it on OS X, it didn’t recognize it.
I reformatted it using OS X and now everything works 🙂
Still curious that every other homebrew app was able to use it and only WiiMC refused it.
