Wii Memory Cards on GameCube Games: Not Compatible

No. Wii uses SD cards (or internal 512 MB NAND) for Wii saves. GameCube games require GameCube Memory Cards (59 / 251 / 1019 blocks) in Slot A/B.

Why

• Different filesystem (FAT12 vs. proprietary).
• Different voltage / pinout.
• Wii's BC (Backward Compatibility) mode exposes only GC memory card slots — SD hidden.

Solutions

• Buy GC Memory Card: $5–15 used (251 block / "Memory Card 59" common).
• GC Memory Card Manager (Homebrew): Backup GC saves to SD → PC → Dolphin. Requires modded Wii.
• Wii U (vWii mode): Same limitation — needs GC card.

Keep a GC card handy if playing GC games on Wii.