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!).
Several people have come to me on Discord and GBAtemp about this. They use both Wii and vWii, and they experience the issues most when using real NAND and the retail disc.
Any help would be greatly appreciated!