U should have used a windows computer that's probably the problem. Some place on this some one asked same thing and they were told to FAT32. And I don't have a mac so I'm assuming it's only for Windows. That is your best bet or change all ur games to GOD files.
This what he said to the other guy.
Second; where do you try to use it? USB? it's also quite possible that your MAC messed up the formatting, you need to have it as MBR, it cannot be GPT (which OSX likes to use)
Get yourself a windows PC and format it there to FAT32, then you can use it with FSD and XEXMenu as long as you put the games on the FAT32 partition and nowhere else...