It's actually quite simple. You probably know that gc disks are optical mediums (this means that they are read and written by optical devices - lasers) and hard disks are magnetic mediums (they are written and read by magnetic heads inside the hard disk case). Unforunately, game cube disks can't be read by any device that pc's support, so you have to do it yourself: Get a good microscope (I use a Digital QX5 -
http://www.compuvisor.com/qx5diblmicom.html ), and take a look at the data stored at the gc disk. Then unscrew you hard disk case, take a small magnet, and copy this pattern on your hard disk. Note that hard disk are very sensitive to dust, so be sure to clean your room before you do that. Normally I can't memorize all the data at once, so I draw some sketches of the data on paper before I do the copying.
Unfortunatly, this is the only way to do that (as documented in other posts in this forum). But it works, as proven by the fact that gc roms exist on the web.
HTH.