Thanks, everyone, for the help! I formatted the drive to MBR using WD Quick Formatter, creating a single partition after backing up the games. Then, I used GUIFormat and formatted the drive to FAT32 with a 64k cluster. after that i moved one GameCube game and one Wii game, they worked flawlessly with no crashes or problems. The entire drive was fully utilized. Now, I'm moving the entire GameCube library back to that disk.
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I tried AOMEI Partition Wizard and EaseUS Partition Master Pro; both are good, but they couldn't do the job. I might indeed try MiniTool Partition next time if I encounter an issue with the disk.
5TB is indeed an overkill!
Again, thanks, everyone, for the help!