There's a Gamecube Homebrew that can copy files off the GC card and to the SD card, you need a SD -> Memory Card Slot adapter though. The prog's called "mcbackup", I think.
Note that, although it has the option of copying back save files and it'll look as if they copied correctly, the game will NOT see the saves copied back using this prog. For copying back use the Wii homebrew provided earlier in this topic, check also its talk page on the wiki.
Edit : I just see it's an old thread, but I leave this anyway for helping people looking for the tool.
I'm using GC homebrew MCbackup, and it's working.
I don't remember trying saves I put back on the GC memory, but I think it's working.
there's maybe few games with their filename different on PC than on memorycard, those one will not work I think.
I've added it to the downloads menu here.
read the description if the Wii is reseting while accessing the SD.
You need to run it with GCOS, then SDload, then MCbackup.dol
Note that it will work with SDGecko (and maybe USBGecko ?)
It doesn't work with Wii Front SD using dol in Wii mode, or HBC.