Okay, to describe my problems more thoroughly, I should note that I'm not the one having this problem.
Let me explain. As some of you here on C3 may have found out already, I distribute custom song software on my GitLab repository Rock Band Customs -- Wii and vWii linked in my signature (some of which I found from people here). I chat with some on GBAtemp and Discord. To guide people step by step, I use Discord. Unfortunately, with at least two people, my customs are not being seen by their systems. In fact, Rock Band 3 doesn't even detect an SD card at all! Yes, they've followed my guides thoroughly, and I'm pretty sure I didn't miss anything, but for those who are curious, you can use my sig links or visit my profile to see my GitLab Wiki guide as well as other guides I left on GBAtemp and Rock Band ScoreHero.
However, I'm asking for help here because I know what a great community this is, and besides guys like StackOverflow0x and Alternity have helped me out a great deal before. When I ask for further help, it probably won't be as long-winded as this one, but I just wanted to let everyone know what I'm doing, and that I am not asking for myself. Customs work just fine for me using my guide, but I'm wondering if I've missed something. Perhaps I figured out something that I forgot to add, or maybe it's a hardware issue (I hope not!).
GBAtemp user riotlancer (Discord warringhearts#9119) is trying this on vWii. GBAtemp user jkitty (Discord jkittymeow#5882) is using a Wii. Both experience the issues when using real NAND and the retail disc; AFAIK they haven't tried other methods.
Any help would be greatly appreciated!