i believe you CANNOT do a backup save on the SD drive
what your essentially doing is loading the game ONTO the SD drive so you can bring it with you everywhere you go in the sd card, HOWEVER it is no longer in your wii's internal memory.
so... you are just pretty much moving your game into the sd card but in no way possible able to move the game from sd back to the wii internal drive.
NOTE:
not really sure if it does 'move' your game from wii to sd or makes a copy, i just wanted to look like i know what i'm saying,

but the rest should be true =] video i saw of someone 'moving' or 'copying' their VC game to an SD had it actually 'moved' since it was no longer in the internal memory screen but showed up only in the SD memory screen, so that is where i assume that it physically 'moves' it and not 'copies'