I tried both my 3DS and 3DSXL, and both fail to boot into Gateway mode. However, running the diagnostics on both systems shows that it's accessing the card (and both systems can read retail carts fine). System's SD cards are both formatted to FAT32, a fresh copy of Launcher.dat (twice tried with different mirrors). Hell, I tried both the paper and the tape trick. It can't be defective, since diagnostics passes, right? Warning: Spoilers inside! Warning: Spoilers inside! Warning: Spoilers inside! Addendum: Tried 3 microSD cards (both exFAT and FAT32), and they all work fine under diagnostics, while simply not having one causes the SD Card portion to fail.