You have to use Cioscorp.
First, ensure you have preloader installed.
Download the softmii or MiiWii package and skip everything and skip right to the cioscorp section.
Once you installed all the wads your Wii will tell you your system menu is corrupt.
Dont worry, just hold reset as you turn it on to boot preloader. Go to the homebrew channel from there and reinstall preloader once again.
Then (assuming you have the right hacks enabled in preloader) backups should run from the disk channel.
I would strongly advise making a nand backup in bootmii first (if you have it installed).
That way if all else fails you know you can just go back to how your wii was before.