GameCube:Creating Game Save Backups

Official
Both the GameCube IPL and the Wii Menu provide basic (and artificially crippled, in the case of protected saves) file management between GameCube memory cards.

GCMM
GCMM is the leading Wii and GameCube homebrew for real save management, with the ability to operate on individual saves, all saves, or the full memory card image. Recent versions added SD2SP2 and even GCLoader storage support.

Swiss
GameCube:Swiss's file management features can, unsurprisingly, be used to copy saves across supported devices.

USB memory card
Third party memory cards with additional USB interface were available. They require proprietary software which might be limited to old/32bit Windows.