actually you can do this.
but you need a hard mod via the internal usb port i think.
i saw a video ages ago with someone hooked up wii to pc, used it as hard drive and to send and boot iso's and apps.
completly useless really though as you can do this over ftp.
easiest way to do a similar thing is to buy a HDD for your Wii - super glue it to your wii (or anything you want) - make sure you have long cable to connect your usb(wii/pc) to tiny usb(HDD)
now when you use your wii plug the hard drive into the usb port on the wii
when you want to use the wii as a hard drive plug your hard drive into your pc
not quite the same but with waninkokos new cios, wad manager, and nand emulator you will achieve the same thing.