Okay folks, now that I've been playing with emuNAND and also been reading some threads, here's what's going on: Your mSDs & SDs aren't bad. They're just not formatted properly. The 3DS uses FAT32 MBR formats. Some flashcarts have special formats like Gateway for emuNAND and Sky3DS for Diskwriter. Your brand-new-super-fast mSD may be out-of-box, but that's doesn't mean it'll work. For any SD to work in the 3DS it -HAS- to be MBR - FAT32 - PRIMARY PARTITION. The problem? Most mSD's (especially SDXC cards) use exFAT. This needs to be changed to FAT32. You can also FAT32 a 128GB SD card and it will work flawlessly in the 3DS. But before you partition your emuNAND or in Diskwriter, IT MUST BE FAT32!!!! The reason for this is how unpartitioning from Diskwriter uses a floppy disk format known as FAT12. They're both FAT file systems. To completely reset the SD you need to jump to NTFS file system and back to FAT32. Otherwise the MBR might not change its values properly. For Diskwriter users, if you get stuck, FORMAT it in Diskwriter. Then use REFORMAT to return it to a workable state. This also goes for folks trying to break their emuNAND formats. Yes, you deleted all your partitions and even re-built the MBR, but there's that one partition with the emuNAND file system data that won't break. To break it? Delete all partitions. Now you should have one giant UNFORMATTED block and the SD should read (MBR). Now format it to NTFS Primary. Gone. Your SD should be in working condition again. Now you need to format it back to FAT32 (just use windows format) and it should be compatible with your 3DS. You can also choose to format it for emuNAND or back through Diskwriter. Remember: Break format by using NTFS PRIMARY. Then Back to FAT32 PRIMARY. If it reads FAT12 or exFAT you're using the wrong file system. SDXC? Format it to FAT32. 128GB? Format it to FAT32. Break it by NTFS and then back to FAT32. Fin. So why does it matter how fast your mSD is? Well, FAT32 can only handle up to 4GB file transfers. So the FAT file system jumps back down to CLASS 10, ignoring the UHS-1 speeds. If you have it working on exFAT, good for you. But there's going to come a point you will have to jump to NTFS and back to FAT32 for it to work again. So don't say I didn't warn you. Stick to FAT32. Use NTFS to break ex/FAT/12/32 and then jump back to FAT32. If you just want to return to defaults, break FAT by using NTFS then use windows format to DEFAULT SETTINGS and it should return to its original file system (exFAT or whatever). Hope this helps.