Wii Backup Fusion Activity
The complete and simple to use backup solution for your Wii games
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dynup
I can confirm what I believe to be the same issue in linux. As I've been discussing over in https://gbatemp.net/threads/wii-backup-managers-in-gnu-linux-wbfs-conversion-issues.508655/ , it turns out that, when not explicitly specified, wit's (used by wbf) default splitting mode is actually "-4g" (4GB), not "-4G" (4GiB). Wii Backup Fusion seems to default to "-4G", and since 4GiB > 4GB, the split files won't copy to FAT32 partitions.
Splitting doesn't work in OS X