Yes, .firm files, I've been doing this a long time. It was .bin now it's .firm... At least I didn't say .dol! lol
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Okay. To be honest, if I want optimal setup, I need to know your setup. Or risk redoing steps. Is your EmuNAND and SysNAND linked? On your internal SD, inside the Nintendo 3ds folder how many folders with random number names are there?
If you have 2 (or more) your NANDS may already be separate. If you change the wallpaper in Luma does it change the wallpaper in GW mode? If NOT just leave the working GW EmuNAND alone and don't move it to SysNAND. Then your GW NAND is setup and all you need is to set up the BootCTR9.firm and configure it to boot Luma.firm and GW.firm (Use the ini data I put in the spoiler)...
And don't use fastboot. It's really crappy. Dangerous. Luma all the way. Only use GW mode for your Red Cart Offline. Use Luma for everything else. You can boot to your EmuNAND in Luma too. I leave the blue cart in my GW 3ds these days. Don't use the red one unless my nephews are here. Then I can throw the kid safe GW cart in to my 3DS. I use one of the dynamic scripts in the All in one pack AIO I was talking about. There's a bunch of useful scripts to swap configurations and stuff. I think InScripted wanted to make it possible to do EVERYTHING without a PC after an initial copy and paste job. So when they are here, I put the GW in, go in to Godmode9, Run a script and it makes GW the default to boot up. I leave the EmuNAND blank and filled up a 64GB SD with all kid friendly stuff. This way I don't have to open the 3DS to swap SD, I only loose 1 GB in my 128GB card inside the 3DS. And until I run the script to make luma default again, The 5 and 7 year olds can't access My stuff on the SysNAND at all. No missing save file because a kid deleted it. And the fact that I have every M rated game and then some in there is cool too.
Couple other things I did was I made a Wallpaper in all red saying "GATEWAY MODE DO NOT UPDATE" and apply it to the GW NAND. I put a PIN on Luma with a message saying Reward If Found with my contact info. I took the SD out completely. Boot in to Luma with no SD (needs boot.firm copied in to SysNAND) and set a password on it. Then I set up a custom message for the pin that says "STOLEN!! DO NOT PAWN! Gamestop Employee, This system is stolen. Contact [contact information] [3ds serial number] REWARD$!!!. I put a theme on the sd-less mode as well with the same message in all red. I take my 3DS everywhere and now if I lose it or someone steals it they will see the Reward message. If they take out the SD to try to get through the PIN it will show STOLEN!. If they throw a blank SD in there it wont boot. I know there are ways around it and there is also 3dsSAFE but this seemed like an easier route. Most people won't know and someone might call me.